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/ingestion/poller.go

@@ -132,7 +132,7 @@ func (p *Poller) pollOnceWithErr(ctx context.Context, src config.SourceConfig) e
 		//      the article is gated, so swap in a Wayback snapshot for both
 		//      Pete (image) and the reader (posted link).
 		meta := FetchArticleMeta(originalURL)
-		paywalled := !meta.Fetched || meta.BodyChars < PaywallBodyThreshold
+		paywalled := !meta.Fetched || meta.Paywalled || meta.BodyChars < PaywallBodyThreshold
 		if paywalled {
 			if snap := ResolveWayback(originalURL); snap != "" {
 				snapMeta := FetchArticleMeta(snap)