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/ai/mod.rs

@@ -7,6 +7,7 @@ pub mod gpu;
 pub mod handlers;
 pub mod insight_chat;
 pub mod insight_generator;
+pub mod persona_chat;
 pub mod llamacpp;
 pub mod llm_client;
 pub mod local_llm;
@@ -32,6 +33,10 @@ pub use handlers::{
     get_available_models_handler, get_insight_handler, get_insight_history_handler,
     get_openrouter_models_handler, rate_insight_handler, turn_async_handler, turn_replay_handler,
 };
+pub use persona_chat::{
+    persona_chat_history_handler, persona_chat_reset_handler, persona_chat_turn_handler,
+    persona_turn_cancel_handler, persona_turn_replay_handler,
+};
 pub use insight_generator::InsightGenerator;
 pub use llamacpp::LlamaCppClient;
 #[allow(unused_imports)]