Fix graceful shutdown ordering and background-job cancellation

[joestump]

Part of #321. Governing: ADR-0013, ADR-0018, SPEC graceful-shutdown REQ-TMO-003/REQ-WG-004/REQ-SIG-004, SPEC metadata-enrichment-pipeline REQ-ENRICH-041. Depends on: #341 (same shutdown path), #324 (shutdown_timeout config). Hotspot: cmd/server/main.go — serialize with #326/#341/#324.

Requirements

• REQ-TMO-003 order inverted: spec says drain goroutines (≤25s) THEN HTTP shutdown (5s); code runs srv.Shutdown first with the full 30s ctx then races wg.Wait() on the same ctx (cmd/server/main.go:637-659) — audit CRITICAL. Fix order or amend the spec (state which in the PR)
• REQ-WG-004: ticker loops and per-user work goroutines share one WaitGroup; spec requires separating them — audit CRITICAL
• REQ-SIG-004: include the received signal attribute in the "shutdown initiated" log line (main.go:617)
• FindSimilarArtistsForAll never checks ctx.Done() and sleeps 500ms per artist uncancellably (internal/services/similar_artists.go:451-476); also note it has no callers — wire it to a scheduler or remove it. And ArtistFindSimilar spawns an untracked goroutine with context.Background() (internal/handlers/artists.go:653-656) — tie to lifecycle
• REQ-ENRICH-041: overlapping metadata ticks are NOT prevented — syncMetadataForUsers spawns per-user goroutines and returns immediately, and its governing comment falsely claims it blocks (cmd/server/main.go:283-338) — audit CRITICAL. Add a per-loop in-flight guard (this also removes the tag-upsert concurrency trigger tracked under EPIC: Unified Tag Taxonomy Completion #303)

Acceptance Criteria

• SIGTERM under load: background work drains inside the budget, exit code 0, correct log attributes
• A long tick provably prevents the next tick from starting (test with fake clock or instrumentation)
• go test -race clean over the shutdown path

Branch

feature/342-graceful-shutdown-ordering

PR Convention

PR title references the story; body ends with Closes #342, references the parent epic and governing artifacts above.

🤖 Posted on behalf of @joestump by Claude.