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/playlists/system_test.go

@@ -24,9 +24,16 @@ func discardLogger() *slog.Logger {
 // `InsertPlayEvent` doesn't accept was_skipped (that's set by an UPDATE).
 func seedPlayEvent(t *testing.T, pool *pgxpool.Pool, userID, trackID pgtype.UUID, startedAt time.Time, wasSkipped bool) {
 	t.Helper()
+	// play_events.session_id has a FK to play_sessions; create a parent
+	// session in the same statement (a random UUID violates the FK).
 	_, err := pool.Exec(context.Background(), `
+		WITH s AS (
+			INSERT INTO play_sessions (user_id, started_at, last_event_at)
+			VALUES ($1, $3, $3)
+			RETURNING id
+		)
 		INSERT INTO play_events (user_id, track_id, session_id, started_at, was_skipped)
-		VALUES ($1, $2, gen_random_uuid(), $3, $4)
+		SELECT $1, $2, s.id, $3, $4 FROM s
 	`, userID, trackID, startedAt, wasSkipped)
 	if err != nil {
 		t.Fatalf("seed play_event: %v", err)