fix: add merge migration to resolve multiple heads from renamed migration

The server has both the old (2026_05_27_make_project_id_nullable) and
new (2026_05_27_external_repos) migration files, creating two heads.
This merge migration resolves them into a single head.

apps/api/alembic/versions/2026_05_27_merge_heads.py

@@ -0,0 +1,27 @@
+"""merge_git_mount_heads
+
+Revision ID: 2026_05_27_merge_heads
+Revises: 2026_05_27_external_repos, 2026_05_27_make_project_id_nullable
+Create Date: 2026-05-27 08:45:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+
+# revision identifiers, used by Alembic.
+revision: str = "2026_05_27_merge_heads"
+down_revision: Union[str, Sequence[str], None] = ("2026_05_27_external_repos", "2026_05_27_make_project_id_nullable")
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # This is a merge migration - no schema changes needed
+    # It resolves the multiple heads created by the renamed migration
+    pass
+
+
+def downgrade() -> None:
+    pass