feat: implement git repository management

- Add backend API for git repository CRUD (create, list, delete)
- Support bare repository initialization and mirror cloning
- Add cascade delete for repositories when project is deleted
- Add frontend page for repository management per project
- Update project page with link to repositories
- Add repo_base_path to config
- Quality gates: ruff, mypy, typecheck, lint, build all pass

openspec/changes/git-repo-management/specs/git-repo-management/spec.md

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+## ADDED Requirements
+
+### Requirement: Repository Creation
+
+The system SHALL allow creating new bare git repositories within projects.
+
+#### Scenario: Create repository
+- GIVEN an authenticated user with a project
+- WHEN they POST a repository name
+- THEN a bare repo is initialized on disk at `/data/repos/{user_id}/{project_id}/{repo_name}.git`
+- AND metadata is stored in the database
+- AND the response includes the repository details
+
+#### Scenario: Duplicate name prevention
+- GIVEN a project with a repo named "frontend"
+- WHEN the user tries to create another "frontend" repo
+- THEN the system responds with 400 Bad Request
+
+### Requirement: Repository Cloning
+
+The system SHALL support cloning external repositories as bare mirrors.
+
+#### Scenario: Clone repository
+- GIVEN an authenticated user with a project
+- WHEN they provide a valid remote URL
+- THEN the system clones as a bare mirror
+- AND stores it in the structured path
+- AND records the remote URL in metadata
+
+#### Scenario: Invalid URL
+- GIVEN an authenticated user
+- WHEN they provide an invalid or unreachable URL
+- THEN the system responds with 400 Bad Request
+
+### Requirement: Repository Listing
+
+The system SHALL list all repositories for a project.
+
+#### Scenario: List repositories
+- GIVEN an authenticated user with a project
+- WHEN they GET the repositories endpoint
+- THEN all repos for that project are listed with name, path, and last push date
+
+### Requirement: Repository Deletion
+
+The system SHALL remove repository records and disk contents.
+
+#### Scenario: Delete repository
+- GIVEN a project owner
+- WHEN they delete a repository
+- THEN the database record is removed
+- AND the directory on disk is removed
+
+### Requirement: Project Cascade Delete
+
+The system SHALL clean up repositories when a project is deleted.
+
+#### Scenario: Cascade repository cleanup
+- GIVEN a project with associated repositories
+- WHEN the project owner deletes the project
+- THEN all repository records for that project are removed
+- AND all repository directories on disk are removed