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/models/system/__init__.py

@@ -0,0 +1,8 @@
+"""System models module."""
+
+from src.models.system.health_check import HealthCheck
+from src.models.system.instance_event import InstanceEvent
+from src.models.system.notification import Notification
+from src.models.system.terminal_session import TerminalSessionModel
+
+__all__ = ["HealthCheck", "InstanceEvent", "Notification", "TerminalSessionModel"]