Admin: create/delete mysteries, asset library, cutscene component picker

- Mysteries tab: "+ New" (slug + title) and per-row delete; opens the graph editor.
- Assets tab: upload images/audio/PDFs to the shared osint.assets store, grid with
  thumbnails/icons, copy id/url, delete (blocked with 409 when the asset is in use).
- Cutscene component_key is now a datalist of registered keys with a
  "not registered" warning, instead of free text.

Backend: DELETE /api/admin/mysteries/:id and POST/GET/DELETE /api/admin/assets
(reusing the deduplicated, MinIO-backed asset store).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/narrative.tsx

@@ -45,6 +45,7 @@ const GlassHarbourDiversion: FC<{ onComplete: () => void }> = ({ onComplete }) =
   </div>
 )
 const CUTSCENE_REGISTRY: Record<string, FC<{ onComplete: () => void }>> = { 'glass-harbour-diversion': GlassHarbourDiversion }
+export const CUTSCENE_COMPONENT_KEYS = Object.keys(CUTSCENE_REGISTRY)
 
 export function CutsceneHost({ componentKey, label, onComplete }: { componentKey: string | null | undefined; label: string; onComplete: () => void }) {
   const Component = componentKey ? CUTSCENE_REGISTRY[componentKey] : undefined