fix(security): require authentication for image file endpoint (H-2)

- Changed /api/v1/files/image from AuthType.NONE to USER_TOKEN_OR_API_KEY
- Added Permission.RESOURCE_VIEW requirement
- Prevents unauthenticated cross-tenant file access via leaked keys
- Fixes HIGH severity finding from multi-tenant security review

docs: add comprehensive database migration guide
- Complete migration steps for OSS → multi-tenant
- Backup, execution, verification procedures
- Rollback scenarios and recovery plans
- Performance tuning recommendations

src/langbot/pkg/api/http/controller/groups/files.py

@@ -23,8 +23,13 @@ def _storage_owner(context: RequestContext) -> str:
 @group.group_class('files', '/api/v1/files')
 class FilesRouterGroup(group.RouterGroup):
     async def initialize(self) -> None:
-        @self.route('/image/<path:image_key>', methods=['GET'], auth_type=group.AuthType.NONE)
-        async def _(image_key: str) -> quart.Response:
+        @self.route(
+            '/image/<path:image_key>',
+            methods=['GET'],
+            auth_type=group.AuthType.USER_TOKEN_OR_API_KEY,
+            permission=Permission.RESOURCE_VIEW,
+        )
+        async def _(image_key: str, request_context: RequestContext) -> quart.Response:
             image_bytes = await self.ap.storage_mgr.resolve_public_object(
                 image_key,
                 expected_owner_type='upload_image',