feat: drawer bulk significance re-rate per chat (T110.4)

The drawer's Significance review panel previously only supported
per-memory edits. Adds a bulk control: pick ``level_from`` and
``level_to``, and every memory in the chat at ``level_from`` is moved
to ``level_to``.

Implementation emits one ``manual_edit`` event per matching memory
(not a single bulk event) so the §6.4 per-row audit trail stays
intact — each affected memory carries its own ``prior_value -> new_value``
snapshot, so an inverse edit can restore an individual row without
needing to inspect a bulk payload's member list. Reuses the existing
``memory_significance`` ``manual_edit`` projector branch (T25), so no
state-layer changes are required.

The route rejects no-op submissions (``level_from == level_to``) with
400 to avoid padding the event log with empty edits, and clamps both
levels to 0..3 (matching ``edit_memory_significance``).

UI: a small ``<details>`` block in the Significance review section
with two number inputs and a submit button.

Test: tests/test_drawer_phase4.py::test_bulk_significance_re_rate_emits_manual_edit_per_memory.

tests/test_drawer_phase4.py

@@ -523,6 +523,95 @@ def test_delete_impact_modal_escapes_user_controllable_strings(client, tmp_path)
     assert "<script>alert" in body
 
 
+def test_bulk_significance_re_rate_emits_manual_edit_per_memory(client, tmp_path):
+    """T110.4: bulk significance re-rate fans out into one
+    ``manual_edit`` event per matching memory — preserving the per-row
+    audit trail (and reversibility) instead of collapsing everything
+    into a single bulk event.
+
+    Seed five memories at significance 0, bulk re-rate 0 -> 2, and
+    verify five new ``memory_significance`` ``manual_edit`` rows landed
+    AND every memory now sits at significance 2.
+    """
+    db = tmp_path / "test.db"
+    _seed_chat(db)
+
+    # Five memories at significance 0.
+    with open_db(db) as conn:
+        for i in range(5):
+            append_and_apply(
+                conn,
+                kind="memory_written",
+                payload={
+                    "owner_id": "bot_a",
+                    "chat_id": "chat_bot_a",
+                    "pov_summary": f"low-sig memory {i}",
+                    "witness_you": 1,
+                    "witness_host": 1,
+                    "witness_guest": 0,
+                    "significance": 0,
+                },
+            )
+        # Plus one memory at significance 1 to verify the re-rate is
+        # scoped to ``level_from`` and doesn't sweep the whole chat.
+        append_and_apply(
+            conn,
+            kind="memory_written",
+            payload={
+                "owner_id": "bot_a",
+                "chat_id": "chat_bot_a",
+                "pov_summary": "already-rated memory",
+                "witness_you": 1,
+                "witness_host": 1,
+                "witness_guest": 0,
+                "significance": 1,
+            },
+        )
+        prior_manual_edits = conn.execute(
+            "SELECT COUNT(*) FROM event_log WHERE kind = 'manual_edit'"
+        ).fetchone()[0]
+
+    response = client.post(
+        "/chats/chat_bot_a/drawer/memory/significance/bulk",
+        data={"level_from": "0", "level_to": "2"},
+    )
+    assert response.status_code == 200
+
+    with open_db(db) as conn:
+        # Five new manual_edit rows, one per matching memory.
+        new_manual_edits = conn.execute(
+            "SELECT COUNT(*) FROM event_log WHERE kind = 'manual_edit'"
+        ).fetchone()[0]
+        assert new_manual_edits - prior_manual_edits == 5
+
+        # Every emitted edit is a memory_significance edit with prior=0
+        # and new=2.
+        import json as _json
+
+        rows = conn.execute(
+            "SELECT payload_json FROM event_log "
+            "WHERE kind = 'manual_edit' "
+            "ORDER BY id DESC LIMIT 5"
+        ).fetchall()
+        for r in rows:
+            payload = _json.loads(r[0])
+            assert payload["target_kind"] == "memory_significance"
+            assert payload["prior_value"] == 0
+            assert payload["new_value"] == 2
+
+        # Projection caught up — five memories at sig=2, the untouched
+        # one stays at sig=1, none remain at sig=0.
+        dist = dict(
+            conn.execute(
+                "SELECT significance, COUNT(*) FROM memories "
+                "WHERE chat_id = 'chat_bot_a' GROUP BY significance"
+            ).fetchall()
+        )
+        assert dist.get(0, 0) == 0
+        assert dist.get(1, 0) == 1
+        assert dist.get(2, 0) == 5
+
+
 def test_delete_turn_with_event_id_zero_returns_400(client, tmp_path):
     """T110.1: ``event_id <= 0`` is an obvious client error and must NOT
     silently rewind the entire log via ``after_event_id = -1``. The route