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/database/schema.rs

@@ -171,6 +171,16 @@ diesel::table! {
     }
 }
 
+diesel::table! {
+    persona_chat_conversations (user_id, persona_id) {
+        user_id -> Integer,
+        persona_id -> Text,
+        messages_json -> Text,
+        turn_count -> Integer,
+        updated_at -> BigInt,
+    }
+}
+
 diesel::table! {
     personas (id) {
         id -> Integer,
@@ -345,6 +355,7 @@ diesel::allow_tables_to_appear_in_same_query!(
     insight_generation_jobs,
     libraries,
     location_history,
+    persona_chat_conversations,
     personas,
     persons,
     photo_insights,