Fix push SSRF, cross-user unsub, and personalization edge cases

Code review of the personalization/feeds/PWA/push work surfaced ten
confirmed issues, now fixed:

- Web Push delivery bypassed the SSRF guard (unguarded default client);
  now routes through safehttp.NewClient with a hard timeout, and the
  subscribe handler validates the endpoint URL.
- Push unsubscribe deleted by endpoint with no owner check; added
  RemovePushSubscriptionForUser scoped to the signed-in user.
- Byte-slice body/content truncation could split a UTF-8 rune and break
  the RSS content:encoded XML; added a rune-safe truncateUTF8 helper.
- Digest sender could permanently starve a user who hid a high-volume
  source; step the watermark past a full hidden-source scan window.
- Service worker cached personalized HTML navigations into a shared
  cache (identity leak across PWA users); navigations are now
  network-only, CACHE_VERSION bumped to v2 to purge stale pages.
- Public /api/article leaked discarded/unclassified bodies; filter to
  classified, non-sentinel stories.
- runLocal never started the push sender; digests now fire in -local.
- Push client had no timeout, so one hung endpoint stalled all sends.
- Reader migration resurrected cross-device-cleared reads; gate it
  behind a one-time flag so the server stays authoritative.
- Bookmarks count didn't match the classified list filter.

main.go

@@ -377,6 +377,10 @@ func runLocal(cfg *config.Config) {
 		os.Exit(1)
 	}
 	go ws.Start(ctx)
+	// Push only needs the web auth layer, not Matrix, so a web-only deployment
+	// still runs the digest sender — otherwise subscriptions accumulate but no
+	// digest ever fires.
+	ws.StartPushSender(ctx)
 	slog.Info("local: web UI listening", "addr", cfg.Web.ListenAddr)
 
 	storage.RunMaintenance()