perf: read_recent_dialogue pushes chat-id filter into SQL (T90.1)

The previous implementation pulled the last N rows in SQL across all
chats and dropped foreign-chat rows in Python. With LIMIT N this could
return far fewer than N relevant rows when other chats had recent
activity. Push the chat_id filter into SQL via json_extract so LIMIT N
always returns N rows scoped to the requested chat.

Test: seeds two chats with 60 turns each interleaved; queries chat_a
with limit=50; asserts exactly 50 chat_a rows returned (was 0 prior to
the fix because chat_b's rows dominated the global tail).

chat/services/turn_common.py

@@ -54,14 +54,21 @@ def read_recent_dialogue(
     regenerate to drop the original assistant_turn from its prompt
     context window before that row has been marked superseded (the
     supersede UPDATE lands at the end so the new event_id is known).
+
+    T90.1: the chat_id filter is pushed into SQL via ``json_extract`` so
+    ``LIMIT N`` always returns N rows scoped to the requested chat. The
+    previous implementation filtered chat_id post-fetch in Python, which
+    let foreign-chat rows fill the LIMIT and yield fewer than N relevant
+    rows in busy multi-chat databases.
     """
     if exclude_event_id is None:
         cur = conn.execute(
             "SELECT id, kind, payload_json FROM event_log "
             "WHERE kind IN ('user_turn', 'user_turn_edit', 'assistant_turn') "
             "  AND superseded_by IS NULL AND hidden = 0 "
+            "  AND json_extract(payload_json, '$.chat_id') = ? "
             "ORDER BY id DESC LIMIT ?",
-            (limit,),
+            (chat_id, limit),
         )
     else:
         cur = conn.execute(
@@ -69,15 +76,14 @@ def read_recent_dialogue(
             "WHERE kind IN ('user_turn', 'user_turn_edit', 'assistant_turn') "
             "  AND id != ? "
             "  AND superseded_by IS NULL AND hidden = 0 "
+            "  AND json_extract(payload_json, '$.chat_id') = ? "
             "ORDER BY id DESC LIMIT ?",
-            (exclude_event_id, limit),
+            (exclude_event_id, chat_id, limit),
         )
     rows = list(reversed(cur.fetchall()))
     out: list[dict] = []
     for row_id, kind, payload_json in rows:
         p = json.loads(payload_json)
-        if p.get("chat_id") != chat_id:
-            continue
         if kind in ("user_turn", "user_turn_edit"):
             out.append(
                 {