feat: support external repositories for git mounts

- Make project_id nullable in git_repositories table (migration)
- Allow external repos not tied to any project
- Update validation to allow user-owned external repos in git mounts
- Add /projects/repositories endpoint to list all user repos
- Update frontend to fetch all user repos for git mount selector
- TypeScript and build pass

apps/web/src/api/git_repositories.ts

@@ -40,6 +40,11 @@ export async function listRepositories(projectId: string): Promise<GitRepository
   return response.data;
 }
 
+export async function listAllUserRepositories(): Promise<GitRepository[]> {
+  const response = await apiClient.get<GitRepository[]>("/projects/repositories");
+  return response.data;
+}
+
 export async function createRepository(
   projectId: string,
   data: GitRepositoryCreate