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previously scanned every non-superseded ``assistant_turn`` row across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",
-            (original_assistant_event_id,),
+            "  AND superseded_by IS NULL "
+            "  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)