[#2079] Backfill commits from checkouts

[alanpeixinho]

Parent

#2079
Schema: #2089
Fetch library: #2090

Management command that fills commits (and parent storage from #2089) using the fetch library from #2090. Incremental: only hashes we already see in checkouts and do not yet have metadata for.

In scope

• Select distinct checkouts.git_commit_hash missing from commits (or stub rows still missing author/metadata).
• Call the fetch library; upsert commits and parent links per the schema chosen in #2089 (stubs if using commit_parents FKs).
• Optionally set checkouts.git_commit_message when it is null and git returned a message.
• Skip and log failures (bad URL, missing object, full pack). The job must not die on one hash.
• Dry-run flag.
• Wire ephemeral git: Compose tmpfs on the service that runs the command/cron; pass the scratch path into the library. Do not write mirrors under BACKEND_VOLUME_DIR.

Job

1. Batch missing hashes (batching left to the implementer).
2. Resolve URL if needed (library).
3. Fetch + parse (library).
4. Upsert; continue on error.

Idempotent: re-runs only work on what’s still missing.

Tests

Command upserts from mocked fetch results. Failures skip the hash and continue. Dry-run writes nothing.