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/library/scanner_test.go

@@ -103,6 +103,17 @@ func TestScanner_Integration(t *testing.T) {
 		t.Errorf("artist sort = [%q, %q], want [Artist X, Artist Y]", artists[0].SortName, artists[1].SortName)
 	}
 
+	// The synthetic MP3s carry only an ID3 tag — no decodable audio —
+	// so ffprobe yields duration 0. The scanner deliberately refuses to
+	// skip zero-duration rows (it re-runs them so a later scan can
+	// backfill duration once probing works), which is orthogonal to the
+	// mtime-based incremental-skip this test covers. Simulate a
+	// normally-probed library so the skip path is actually exercised;
+	// updated_at is left untouched (still ≥ file mtime).
+	if _, err := pool.Exec(ctx, "UPDATE tracks SET duration_ms = 1000 WHERE duration_ms = 0"); err != nil {
+		t.Fatalf("seed durations: %v", err)
+	}
+
 	stats2, err := scanner.Scan(ctx, nil)
 	if err != nil {
 		t.Fatalf("second scan: %v", err)