feat: multi-session terminal backend core (PR 1)

- Add TerminalSessionModel DB table with instance_id FK, name, status,
  created_at, last_activity_at, closed_at columns
- Add Alembic migration for terminal_sessions table
- Refactor TerminalManager to use composite key (instance_id, session_id)
  supporting up to 5 concurrent sessions per instance
- Add create_session, get_session, get_sessions_for_instance, close_session
- Preserve get_or_create_session for backward compatibility (default session)
- Fix attach_websocket to only close sockets within same session
- Add name (auto-generated 'Session N') and status tracking to TerminalSession
- Add 7 unit tests for multi-session logic

Quality gates: pytest (7 new passed, 174 total passed, 51 pre-existing failures)

apps/api/src/main.py

@@ -24,6 +24,7 @@ from src.api.tool_types import router as tool_types_router
 from src.api.user_config import router as user_config_router
 from src.api.users import router as users_router
 from src.config import Settings
+from src.models.terminal_session import TerminalSessionModel  # noqa: F401 – Alembic model discovery
 from src.database import init_database
 from src.logging_config import (
     ExceptionLoggingMiddleware,
@@ -66,7 +67,9 @@ def _sanitize_validation_errors(errors):
             "type": error.get("type"),
             "loc": error.get("loc"),
             "msg": error.get("msg"),
-            "input": str(error.get("input")) if error.get("input") is not None else None,
+            "input": str(error.get("input"))
+            if error.get("input") is not None
+            else None,
         }
         # Convert ctx to safe format
         ctx = error.get("ctx")
@@ -110,10 +113,12 @@ async def on_startup():
     if not db_ready:
         logger.error("Database initialization failed. Shutting down.")
         import sys
+
         sys.exit(1)
 
     logger.info("Startup complete.")
 
+
 app.include_router(health_router)
 app.include_router(auth_router)
 app.include_router(dashboard_router)