feat(server/m7-coverage-gauge): GET /api/admin/library/coverage

New admin handler returns the library-wide cover-art coverage rollup
{total, with_art, pending, settled, pending_no_mbid} for the admin
dashboard gauge. Always 200; zeros on empty library. Same RequireAdmin
middleware as /api/admin/scan/status. Lives under a new /library/
admin sub-namespace, parallel to /scan/ (per-run state) and /covers/
(actions).

Tests gated on MINSTREL_TEST_DATABASE_URL: empty library returns all
zeros, mixed rows produce correct bucket math (verifying the
with_art + pending + settled = total invariant), non-admin returns 403.

internal/api/admin_coverage.go

@@ -0,0 +1,38 @@
+package api
+
+import (
+	"net/http"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+)
+
+// coverageRollupResp is the wire shape for GET /api/admin/library/coverage.
+// All fields are non-negative counts. with_art + pending + settled = total;
+// pending_no_mbid is a subset of pending.
+type coverageRollupResp struct {
+	Total         int64 `json:"total"`
+	WithArt       int64 `json:"with_art"`
+	Pending       int64 `json:"pending"`
+	Settled       int64 `json:"settled"`
+	PendingNoMbid int64 `json:"pending_no_mbid"`
+}
+
+// handleGetLibraryCoverage implements GET /api/admin/library/coverage.
+// Surfaces a library-wide rollup of albums.cover_art_source for the admin
+// dashboard gauge. Always 200; zeros on an empty library.
+func (h *handlers) handleGetLibraryCoverage(w http.ResponseWriter, r *http.Request) {
+	q := dbq.New(h.pool)
+	row, err := q.GetAlbumCoverageRollup(r.Context())
+	if err != nil {
+		h.logger.Error("admin: get coverage rollup", "err", err)
+		writeErr(w, http.StatusInternalServerError, "server_error", "lookup failed")
+		return
+	}
+	writeJSON(w, http.StatusOK, coverageRollupResp{
+		Total:         row.Total,
+		WithArt:       row.WithArt,
+		Pending:       row.Pending,
+		Settled:       row.Settled,
+		PendingNoMbid: row.PendingNoMbid,
+	})
+}