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.

migrations/2026-08-21-000000_persona_chat/down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS persona_chat_conversations;

migrations/2026-08-21-000000_persona_chat/up.sql

@@ -0,0 +1,22 @@
+-- Open chat with persona — file-anchored insight chat is keyed by
+-- (library_id, file_path). The open chat is keyed by (user_id, persona_id)
+-- with a single rolling transcript per pair. No tree branching in v1
+-- (matches the locked-in "single rolling conversation per persona" scope);
+-- a flat JSON blob is enough and avoids forcing a tree shape onto a
+-- surface that's intentionally linear.
+--
+-- `messages_json` is the same `Vec<ChatMessage>` shape the file-anchored
+-- chat persists, so the SSE replay / UI rendering on the mobile client
+-- can reuse the same parser without a second schema.
+
+CREATE TABLE persona_chat_conversations (
+    user_id INTEGER NOT NULL,
+    persona_id TEXT NOT NULL,
+    messages_json TEXT NOT NULL DEFAULT '[]',
+    turn_count INTEGER NOT NULL DEFAULT 0,
+    updated_at BIGINT NOT NULL,
+    PRIMARY KEY (user_id, persona_id)
+);
+
+CREATE INDEX idx_persona_chat_updated
+    ON persona_chat_conversations (user_id, updated_at DESC);