feat: add /persona_chat endpoints for open chat with persona

- PersonaChatSession dispatches turns through the shared agent loop,
  persisting transcripts keyed on (user_id, persona_id) in the new
  persona_chat table (migration included).
- GET /persona_chat/history returns a rendered transcript (tool
  invocations folded, is_initial flag) matching the file-chat shape.
- POST /persona_chat/turn returns 202 with a turn_id; SSE replay and
  cancel reuse turn_replay_impl/cancel_turn_impl, extracted from the
  actix-attributed handlers so both route families share the logic.
- POST /persona_chat/reset clears the persona transcript.
- Concurrent dispatches for the same (user, persona) are rejected with
  409 via an in-flight gate (InFlightPersonaTurns) whose RAII guard
  drops with the spawned turn task, freeing the slot on completion,
  error, or abort.

src/main.rs

@@ -382,6 +382,11 @@ fn main() -> std::io::Result<()> {
                 .service(ai::turn_async_handler)
                 .service(ai::turn_replay_handler)
                 .service(ai::cancel_turn_handler)
+                .service(ai::persona_chat_history_handler)
+                .service(ai::persona_chat_turn_handler)
+                .service(ai::persona_chat_reset_handler)
+                .service(ai::persona_turn_replay_handler)
+                .service(ai::persona_turn_cancel_handler)
                 .service(ai::rate_insight_handler)
                 .service(ai::export_training_data_handler)
                 .service(ai::tts_speech_handler)