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/models/__init__.py

@@ -4,9 +4,23 @@ from src.models.config_profile import ConfigProfile, ConfigProfileInclude
 from src.models.git_repository import GitRepository
 from src.models.project import Project
 from src.models.ssh_key import SSHKey
+from src.models.terminal_session import TerminalSessionModel
 from src.models.tool_instance import ToolInstance
 from src.models.tool_type import ToolType
 from src.models.user import User
 from src.models.user_config import UserConfig
 
-__all__ = ["Base", "ConfigFolder", "ConfigProfile", "ConfigProfileInclude", "GitRepository", "Project", "SSHKey", "ToolInstance", "ToolType", "User", "UserConfig"]
+__all__ = [
+    "Base",
+    "ConfigFolder",
+    "ConfigProfile",
+    "ConfigProfileInclude",
+    "GitRepository",
+    "Project",
+    "SSHKey",
+    "TerminalSessionModel",
+    "ToolInstance",
+    "ToolType",
+    "User",
+    "UserConfig",
+]