test: fix 4 integration-suite root causes surfaced by CI (C+A+B+F)

Latent failures exposed now that integration tests run in CI (#339).
All test/test-harness only — no production code changes.

A (dbtest.ResetDB): also truncate cover_art_provider_settings +
cover_art_sources_meta. The monotonic source-version counter (seeded 1
by 0018) accumulated across internal/coverart tests → CurrentVersion=4
want 1, key-only-bump assertions, enricher source skips. SettingsService
re-seeds both at boot, so a truncated start is the correct fresh state.

B (playlists system_test.seedPlayEvent): inserted play_events with a
random session_id → play_events_session_id_fkey violation (7 tests).
Create the parent play_sessions row in the same statement (CTE).

C (similarity worker_integration_test.newTestWorker): built the client
with only BaseURL. Post-4fca0e6 similarity hits the Labs API via
LabsBaseURL, so an unset LabsBaseURL fell through to the real labs.api
and the stub never ran → 0 similarity rows. Set LabsBaseURL to the stub.

F (library scanner_test): NOT a flake — deterministic. Synthetic
ID3-only MP3s have no decodable duration; the scanner intentionally
won't skip duration_ms=0 rows (retries duration backfill), so every
re-scan reported Updated not Skipped. Seed duration_ms>0 before the
second scan so the mtime-based incremental-skip path under test is
actually exercised. Production scanner behavior unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/similarity/worker_integration_test.go

@@ -114,8 +114,11 @@ func markPlayed(t *testing.T, f fixture, trackID pgtype.UUID) {
 func newTestWorker(f fixture, lbBaseURL string) *Worker {
 	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
 	return &Worker{
-		pool:   f.pool,
-		client: &listenbrainz.Client{BaseURL: lbBaseURL, HTTP: http.DefaultClient},
+		pool: f.pool,
+		// LabsBaseURL must point at the stub too: similarity calls hit
+		// the Labs API (commit 4fca0e6), so an unset LabsBaseURL would
+		// fall through to the real labs.api and the stub never runs.
+		client: &listenbrainz.Client{BaseURL: lbBaseURL, LabsBaseURL: lbBaseURL, HTTP: http.DefaultClient},
 		logger: logger,
 		tick:   1 * time.Hour,
 		batch:  5,