fix(terminal): prevent double session creation, restore sessions on reload

Three fixes for multi-session terminal bugs:

1. Race-condition double creation: The auto-create effect fired twice because
   loadSessions returned 0 while an earlier createSession was still in flight.
   Added hasAutoCreated guard ref to ensure only one auto-create happens.

2. Page reload spawns new sessions: After server restart, list_terminal_sessions
   filtered out DB-only sessions (no in-memory counterpart), so the frontend
   thought no sessions existed and auto-created new ones. Reverted the filter
   so DB rows are always returned. The WebSocket handler now restores the
   in-memory session from the DB row on demand when connecting.

3. No input after connection: The backend _read_loop would break on any send
   error, causing asyncio.wait to cancel the _write_loop. Made _read_loop
   retry up to 3 times before giving up, preventing transient send errors
   from killing input handling.

Quality gates: pytest (15/15 passed), tsc clean

apps/api/src/services/terminal_manager.py

@@ -131,6 +131,7 @@ class TerminalManager:
         container_id: str,
         startup_command: str | None = None,
         name: str | None = None,
+        session_id: str | None = None,
     ) -> TerminalSession:
         """Create a new terminal session for an instance.
 
@@ -159,7 +160,8 @@ class TerminalManager:
                 instance_id_str, self.MAX_SESSIONS_PER_INSTANCE
             )
 
-        session_id = str(uuid.uuid4())
+        if session_id is None:
+            session_id = str(uuid.uuid4())
         session = TerminalSession(
             session_id=session_id,
             instance_id=instance_id,
@@ -172,7 +174,7 @@ class TerminalManager:
         key = (instance_id_str, session_id)
         self._sessions[key] = session
 
-        # Fire-and-forget DB insert
+        # Fire-and-forget DB insert (skip if row already exists)
         asyncio.create_task(
             self._insert_db_session_row(session_id, instance_id, session.name)
         )