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/dbtest/reset.go

@@ -55,6 +55,14 @@ var dataTables = []string{
 	"playlist_tracks",
 	"playlists",
 	"library_changes", // M7 #357 — must reset to keep cursor isolated per test
+	// Cover-art provider settings + the monotonic source-version
+	// counter (cover_art_sources_meta, seeded at 1 by migration 0018).
+	// Not resetting these let the version accumulate across tests in
+	// internal/coverart (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.
+	"cover_art_provider_settings",
+	"cover_art_sources_meta",
 	"tracks",
 	"albums",
 	"artists",