Paywall signal detection, sqlite driver registration, /img route fix

- Detect explicit paywall markers (article:content_tier meta, JSON-LD
  isAccessibleForFree) so metered articles fall back to Wayback even
  when body length is above threshold
- Blank-import go.mau.fi/util/dbutil/litestream so the sqlite3-fk-wal
  driver mautrix's cryptohelper depends on is registered
- Fix /img route: Go ServeMux requires {wildcard} to be a whole segment,
  so capture {name} and strip the .avif suffix in the handler

internal/web/thumbs.go

@@ -56,7 +56,12 @@ func thumbURL(src string) string {
 }
 
 func (s *Server) handleImg(w http.ResponseWriter, r *http.Request) {
-	hash := r.PathValue("hash")
+	name := r.PathValue("name")
+	hash, ok := strings.CutSuffix(name, ".avif")
+	if !ok {
+		http.NotFound(w, r)
+		return
+	}
 	src := r.URL.Query().Get("u")
 	if src == "" || thumbKey(src) != hash {
 		http.NotFound(w, r)