fix: resolve mapper configuration for ToolDefinitionManifest and Workspace

Move ToolDefinitionManifest import out of TYPE_CHECKING in tool_type.py
so SQLAlchemy can resolve the string-annotated relationship during mapper
configuration.

Add Workspace to models/__init__.py (before ToolInstance) so the
ToolInstance-Workspace relationship can be resolved.

Quality gates: py_compile passed, ruff passed, all mappers configure OK.

apps/api/src/models/__init__.py

@@ -12,6 +12,7 @@ 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
+from src.models.workspace import Workspace
 
 __all__ = [
     "Base",
@@ -29,4 +30,5 @@ __all__ = [
     "ToolType",
     "User",
     "UserConfig",
+    "Workspace",
 ]

apps/api/src/models/tool_type.py

@@ -7,8 +7,9 @@ from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from src.models.base import Base, TimestampMixin, UUIDPrimaryKeyMixin
 
+from src.models.tool_definition_manifest import ToolDefinitionManifest
+
 if TYPE_CHECKING:
-    from src.models.tool_definition_manifest import ToolDefinitionManifest
     from src.models.user import User