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/api.go

@@ -114,6 +114,8 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 
 				admin.Get("/scan/status", h.handleGetScanStatus)
 				admin.Post("/scan/run", h.handleTriggerScan)
+
+				admin.Get("/library/coverage", h.handleGetLibraryCoverage)
 			})
 
 			authed.Get("/playlists", h.handleListPlaylists)