feat(api): port repair_audio_analyses backfill job

[raymondjacobson]

Summary

Adds a recurring job that backfills missing track bpm / musical_key from content-node audio analyses — a direct port of apps' discovery-provider repair_audio_analyses celery task (3-minute beat). This is the parity-correct backfill mechanism for tracks left without bpm/key (companion to the indexer fix in go-openaudio#334).

Each pass:

• Selects up to 1000 current tracks missing a non-custom bpm or musical_key, with audio_analysis_error_count < 3, a streamable track_cid, non-podcast/audiobook genre, newest first.
• Picks up to 5 random registered content-node endpoints via Eth.GetRegisteredEndpoints.
• For each track, queries /uploads/{audio_upload_id} (modern) or /tracks/legacy/{track_cid}/analysis (legacy, when audio_upload_id is empty), falling back to the next node on transport/non-2xx error.
• Fills in any missing and valid bpm / musical_key and syncs the stored error count, committing per track.

Validation mirrors apps: valid_bpm is a float in (0, 999); valid musical keys come from the flats-only MusicalKey enum. Both lowercase (mediorum) and capitalized (legacy python content node) JSON keys are accepted.

Changes

• jobs/repair_audio_analyses.go: the job (query → node discovery → per-track fetch/apply).
• jobs/musical_key.go: isValidMusicalKey + the enum map.
• jobs/repair_audio_analyses_test.go: unit tests.
• indexer/indexer.go: register in startParityJobs (every 3 min), passing the SDK for content-node discovery.

Test plan

• go build ./jobs/... ./indexer/... + go vet ./jobs/ clean
• Unit tests pass: bpm/key validation, lowercase+capitalized JSON parsing, modern/legacy endpoint parsing, non-2xx fallback, empty-results handling
• Observe on staging: confirm tracks with NULL bpm/key get populated and error counts advance

🤖 Generated with Claude Code

[raymondjacobson]

Bumped go-openaudio + pkg/etl from 5ed068b → dca79f0 to consume the now-merged indexer fix (OpenAudio/go-openaudio#334). The vendored ETL indexer now persists track bpm/musical_key/audio_upload_id, which is the prerequisite for this backfill job (it keys the modern uploads endpoint on audio_upload_id). go mod tidy, full go build ./..., and the jobs tests pass on the bumped deps.