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/api/admin_coverart_research.go

@@ -0,0 +1,22 @@
+package api
+
+import "net/http"
+
+// handleResearchMissingArt bumps the cover-art version unconditionally
+// so every 'none' row becomes eligible at the next enrichment pass.
+// Operator's escape hatch when adding a Last.fm key, when an upstream
+// provider's catalog has updated, or when verifying a fix without
+// waiting for the next scheduled scan.
+//
+// Existing positively-sourced rows (sidecar/embedded/mbcaa/theaudiodb/
+// deezer/lastfm) are not affected — the version-mismatch eligibility
+// rule only kicks in for 'none'.
+func (h *handlers) handleResearchMissingArt(w http.ResponseWriter, r *http.Request) {
+	newVer, err := h.coverSettings.BumpVersion(r.Context())
+	if err != nil {
+		h.logger.Error("admin: research missing art bump failed", "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "bump failed")
+		return
+	}
+	writeJSON(w, http.StatusOK, map[string]any{"version": newVer})
+}