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/server.go

@@ -69,7 +69,7 @@ func New(cfg config.WebConfig) (*Server, error) {
 		return nil, err
 	}
 	mux.Handle("GET /static/", http.StripPrefix("/static/", http.FileServer(http.FS(staticSub))))
-	mux.HandleFunc("GET /img/{hash}.avif", s.handleImg)
+	mux.HandleFunc("GET /img/{name}", s.handleImg)
 
 	mux.HandleFunc("GET /{$}", s.handleIndex)
 	for _, ch := range channels {