fix: address code review findings

- Honor ignore_singles/ignore_compilations at scanner read time so toggles
  take effect immediately on the dashboard, artist page, and digest instead
  of waiting for the MusicBrainz cache to expire and prune rows.
- Run notifier notify synchronously in the scheduler loop to avoid overlapping
  read-send-mark runs double-sending the digest.
- Show artist name (with ID fallback) on the archive page instead of raw IDs.
- Select last_synced in GetAllArtistSettings for contract consistency.
- Fix stale startPeriodicSync comment and remove redundant error var.
- Remove dead ignored-branch from the artist template (never rendered).
- Add tests: CSRF sameOrigin, ArtistCacheFresh, secondary_types round-trip,
  and scanner type-toggle filtering.
- Update Specification.md schema/config to reflect mbid, last_synced,
  secondary_types, sync.interval, and server.public_url.

internal/web/templates/archive.html

@@ -27,7 +27,7 @@
         <tbody>
         {{ range .Releases }}
             <tr>
-                <td>{{ .ArtistID }}</td>
+                <td>{{ if .ArtistName }}{{ .ArtistName }}{{ else }}{{ .ArtistID }}{{ end }}</td>
                 <td>{{ .Title }}</td>
                 <td>{{ if .Type }}{{ .Type }}{{ else }}<span class="empty">—</span>{{ end }}</td>
                 <td>{{ if .ReleaseDate }}{{ .ReleaseDate }}{{ else }}<span class="empty">—</span>{{ end }}</td>