perf: scope regenerate sibling-lookup to chat_id (T83.3)

The sibling assistant_turn lookup in ``regenerate_assistant_turn``
previously scanned every non-superseded ``assistant_turn`` row across
the whole database and filtered in Python. With many chats in the log
this is O(total_assistant_turns) per regenerate.

Push the chat_id filter into SQL via ``json_extract(payload_json,
'$.chat_id') = ?`` and add ``ORDER BY id DESC LIMIT 50`` so worst-case
work is bounded even within a single chat. Mirrors the SQL pattern in
``chat.web.meanwhile._last_meanwhile_speaker``.

Test added: test_regenerate_sibling_lookup_scoped_to_chat seeds two
chats — the second has an interjection whose ``interjection_of`` value
collides with the first chat's primary speaker. Regenerating chat A
must leave chat B's rows untouched and the regenerated chat A
interjection's ``regenerated_from`` must point at chat A's original
(not chat B's). Pre-T83.3 a global query could in principle latch
onto cross-chat rows.

chat/services/regenerate.py

@@ -148,6 +148,13 @@ async def regenerate_assistant_turn(
     # the silent witness (the bot that wasn't the primary addressee).
     # Filter on ``superseded_by IS NULL`` so prior regenerates of this
     # group don't reappear as siblings.
+    #
+    # T83.3: push the chat_id filter into SQL via ``json_extract`` so
+    # the query doesn't scan every assistant_turn row across the whole
+    # database. ``LIMIT 50`` bounds worst-case work even when chat_id
+    # isn't selective (e.g. a single chat with many turns) — we only
+    # need the one matching sibling. Mirrors the SQL pattern in
+    # ``chat.web.meanwhile._last_meanwhile_speaker``.
     original_interjection_event_id: int | None = None
     original_interjection_payload: dict | None = None
     if original_user_turn_id is not None:
@@ -155,8 +162,11 @@ async def regenerate_assistant_turn(
             "SELECT id, payload_json FROM event_log "
             "WHERE kind = 'assistant_turn' "
             "  AND id != ? "
-            "  AND superseded_by IS NULL",
+            "  AND superseded_by IS NULL "
-            (original_assistant_event_id,),
+            "  AND json_extract(payload_json, '$.chat_id') = ? "
+            "ORDER BY id DESC "
+            "LIMIT 50",
+            (original_assistant_event_id, chat_id),
         )
         for sib_id, sib_payload_json in sibling_cur.fetchall():
             sib_payload = json.loads(sib_payload_json)

tests/test_regenerate.py

@@ -664,6 +664,182 @@ def test_regenerate_drops_interjection_when_classifier_returns_false(
         assert "interjection_of" not in new_primary_payload
 
 
+def test_regenerate_sibling_lookup_scoped_to_chat(tmp_path, monkeypatch):
+    """T83.3: regenerate's sibling-interjection lookup is scoped to the
+    chat being regenerated.
+
+    Setup: TWO chats, each with a primary + interjection turn group whose
+    rows happen to share the same ``user_turn_id`` value (the projector
+    assigns event_log ids monotonically across the whole database, so
+    when each chat is seeded back-to-back the chat A primary lands on a
+    different ``user_turn_id`` than chat B's — but in older versions the
+    sibling query had no chat predicate, so it could in principle latch
+    onto a row from a different chat if ids collided in some unusual
+    flow). We construct the seeding so chat B's interjection has the
+    SAME ``interjection_of`` value as the chat A primary's speaker_id —
+    pre-T83.3 the global query could have picked it up.
+
+    Assert: regenerating the chat A primary leaves chat B's rows
+    untouched (no supersede), and the regenerated chat A turn group's
+    interjection (the only one regenerate should regenerate) has its
+    ``regenerated_from`` pointing at the chat A original interjection,
+    not chat B's.
+    """
+    import asyncio
+
+    from chat.config import Settings
+    from chat.db.migrate import apply_migrations
+    from chat.services import regenerate as regenerate_module
+    from chat.services.interjection import InterjectionDecision
+    from chat.services.regenerate import regenerate_assistant_turn
+
+    db_path = tmp_path / "test.db"
+    cfg = tmp_path / "config.toml"
+    cfg.write_text('featherless_api_key = "test"\n')
+    monkeypatch.setenv("CHAT_CONFIG_PATH", str(cfg))
+    monkeypatch.setenv("CHAT_DB_PATH", str(db_path))
+    apply_migrations(db_path)
+
+    # Seed chat A's interjection group.
+    a_ut_id, a_primary_id, a_interjection_id = _seed_with_interjection_group(
+        db_path
+    )
+
+    # Seed chat B with the same shape but a different chat_id and bot
+    # ids, then add an interjection group whose ``interjection_of``
+    # points at "bot_a" so a global (unscoped) query could collide.
+    with open_db(db_path) as conn:
+        for bot_id, name in (("bot_c", "BotC"), ("bot_d", "BotD")):
+            append_event(
+                conn,
+                kind="bot_authored",
+                payload={
+                    "id": bot_id,
+                    "name": name,
+                    "persona": "",
+                    "voice_samples": [],
+                    "traits": [],
+                    "backstory": "",
+                    "initial_relationship_to_you": "",
+                    "kickoff_prose": "",
+                },
+            )
+        append_event(
+            conn,
+            kind="chat_created",
+            payload={
+                "id": "chat_other",
+                "host_bot_id": "bot_c",
+                "guest_bot_id": "bot_d",
+                "initial_time": "2026-04-26T20:00:00+00:00",
+                "narrative_anchor": "Day 1",
+                "weather": "",
+            },
+        )
+        b_ut_id = append_event(
+            conn,
+            kind="user_turn",
+            payload={
+                "chat_id": "chat_other",
+                "prose": "different chat",
+                "segments": [],
+            },
+        )
+        b_primary_id = append_event(
+            conn,
+            kind="assistant_turn",
+            payload={
+                "chat_id": "chat_other",
+                "speaker_id": "bot_c",
+                "text": "Other primary.",
+                "truncated": False,
+                "user_turn_id": b_ut_id,
+            },
+        )
+        # The chat B interjection's ``interjection_of`` references
+        # "bot_a" — the chat A primary's speaker. Pre-T83.3 the global
+        # sibling query could mis-match this row.
+        b_interjection_id = append_event(
+            conn,
+            kind="assistant_turn",
+            payload={
+                "chat_id": "chat_other",
+                "speaker_id": "bot_d",
+                "text": "Cross-chat noise.",
+                "truncated": False,
+                "user_turn_id": b_ut_id,
+                "interjection_of": "bot_a",
+            },
+        )
+
+    # Stub the interjection classifier to return True so the regenerate
+    # actively walks the sibling-discovery path.
+    async def _stub_should_interject(*_args, **_kwargs):
+        return InterjectionDecision(should_interject=True, reason="fired")
+
+    monkeypatch.setattr(
+        regenerate_module, "detect_interjection", _stub_should_interject
+    )
+
+    state_canned = json.dumps(
+        {"affinity_delta": 0, "trust_delta": 0, "knowledge_facts": []}
+    )
+    canned: list[str] = (
+        ["New chat A primary."]
+        + [state_canned] * 6
+        + ["New chat A interjection."]
+        + [state_canned] * 6
+    )
+    mock_client = MockLLMClient(canned=list(canned))
+    settings = Settings(featherless_api_key="test")
+
+    with open_db(db_path) as conn:
+        new_text = asyncio.run(
+            regenerate_assistant_turn(
+                conn,
+                mock_client,
+                settings=settings,
+                chat_id="chat_multi",
+                original_assistant_event_id=a_primary_id,
+            )
+        )
+        assert new_text == "New chat A primary."
+
+        # Chat B rows are untouched — neither superseded nor referenced.
+        b_primary_super = conn.execute(
+            "SELECT superseded_by FROM event_log WHERE id = ?",
+            (b_primary_id,),
+        ).fetchone()[0]
+        b_interjection_super = conn.execute(
+            "SELECT superseded_by FROM event_log WHERE id = ?",
+            (b_interjection_id,),
+        ).fetchone()[0]
+        assert b_primary_super is None
+        assert b_interjection_super is None
+
+        # Chat A's regenerated interjection has its ``regenerated_from``
+        # pointing at chat A's original interjection — NOT chat B's.
+        cur = conn.execute(
+            "SELECT payload_json FROM event_log "
+            "WHERE kind = 'assistant_turn' "
+            "  AND id NOT IN (?, ?, ?, ?) "
+            "  AND superseded_by IS NULL",
+            (a_primary_id, a_interjection_id, b_primary_id, b_interjection_id),
+        ).fetchall()
+        # Two new rows: regenerated primary + regenerated interjection.
+        assert len(cur) == 2
+        payloads = [json.loads(row[0]) for row in cur]
+        # Find the regenerated interjection (carries interjection_of).
+        new_interject_payloads = [
+            p for p in payloads if p.get("interjection_of")
+        ]
+        assert len(new_interject_payloads) == 1
+        assert new_interject_payloads[0]["regenerated_from"] == a_interjection_id
+        # Pin chat scope on every new row.
+        for p in payloads:
+            assert p["chat_id"] == "chat_multi"
+
+
 def test_regenerate_registers_task_in_in_flight_tasks(tmp_path, monkeypatch):
     """T83.1: regenerate's streaming Task is registered in the chat-keyed
     ``_in_flight_tasks`` dict so the /turns/cancel route can cancel a