feat(server/web/coverart): manual Re-search missing art button

POST /api/admin/cover-sources/research bumps cover_art_sources_meta.
current_version unconditionally; the next enrichment pass re-eligibles
every 'none' row. Existing positively-sourced rows are not affected —
the version-mismatch eligibility rule only kicks in for 'none'.

Admin Cover Art panel gains a "Re-search missing art" button that
calls the endpoint and shows a confirmation toast. Operator's escape
hatch when:

- They've added a Last.fm key and want to retry failed rows
  immediately rather than waiting for the next scheduled scan.
- An upstream provider's catalog has updated (e.g. Deezer added a
  back-catalog import).
- They want to verify a fix end-to-end before the next scheduled
  scan fires.

SettingsService.BumpVersion (the unconditional version of T5's
BumpVersionIfProvidersChanged) is the single-call DB writer; both
endpoints route through it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/coverart/settings.go

@@ -311,6 +311,29 @@ func registeredProvidersHash() string {
 	return hex.EncodeToString(sum[:])
 }
 
+// BumpVersion unconditionally increments cover_art_sources_meta.current_version
+// and returns the new value. Used by the manual "Re-search missing art"
+// admin endpoint when the operator wants every 'none' row re-attempted
+// on the next enrichment pass.
+//
+// Distinct from BumpVersionIfProvidersChanged: that method is the boot-
+// time auto-bump and skips the DB write when the registered-provider
+// hash matches. This method always writes.
+func (s *SettingsService) BumpVersion(ctx context.Context) (int32, error) {
+	q := dbq.New(s.pool)
+	// Reuse BumpVersionAndSetProvidersHash with the current registered-
+	// providers hash; this keeps the stored hash consistent (a manual
+	// bump is conceptually equivalent to "force the auto-bump path").
+	newVer, err := q.BumpVersionAndSetProvidersHash(ctx, registeredProvidersHash())
+	if err != nil {
+		return 0, fmt.Errorf("bump version: %w", err)
+	}
+	s.mu.Lock()
+	s.currentVersion = newVer
+	s.mu.Unlock()
+	return newVer, nil
+}
+
 // TestProvider invokes TestConnection on the named provider if it
 // implements TestableProvider. Returns ErrNotTestable if the
 // provider doesn't, ErrProviderNotFound if not registered.