refactor: reorganize models into subpackages

Move models into domain subpackages (max 4 files each):
- models/tool/ — tool_type, tool_instance, tool_definition_manifest
- models/config/ — config_profile
- models/user/ — user, user_config, ssh_key
- models/project/ — project, git_repository, workspace
- models/system/ — health_check, notification, instance_event, terminal_session

models/__init__.py continues to re-export all symbols, so consumers
using 'from src.models import X' are unaffected. Updated direct file
imports across the backend to use the new paths.

Quality gates: py_compile passed, ruff passed.

apps/api/src/services/notification_service.py

@@ -8,7 +8,7 @@ from sqlalchemy import func, select, update
 from sqlalchemy.engine import CursorResult
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from src.models.notification import Notification
+from src.models import Notification
 
 
 class NotificationService: