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/api/alembic/versions/2026_05_27_make_project_id_nullable.py

@@ -0,0 +1,37 @@
+"""make_project_id_nullable_in_git_repositories
+
+Revision ID: 2026_05_27_make_project_id_nullable
+Revises: e7adfb4
+Create Date: 2026-05-27 08:30:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = "2026_05_27_make_project_id_nullable"
+down_revision: Union[str, Sequence[str], None] = "2026_05_26_add_git_mounts"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Make project_id nullable to allow external repositories
+    op.alter_column(
+        "git_repositories",
+        "project_id",
+        existing_type=sa.UUID(),
+        nullable=True,
+    )
+
+
+def downgrade() -> None:
+    op.alter_column(
+        "git_repositories",
+        "project_id",
+        existing_type=sa.UUID(),
+        nullable=False,
+    )