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.

internal/web/server.go

@@ -59,6 +59,7 @@ type Server struct {
 	tpls           map[string]*template.Template // keyed by page name (e.g. "index", "channel")
 	auth           *Authenticator                // nil when sign-in is disabled or unavailable
 	adminSubs      map[string]bool               // OIDC subjects allowed to view /status
+	pushHTTP       *http.Client                  // SSRF-guarded client for Web Push delivery; built lazily by pushClient()
 
 	// Daily-rotated salt for the privacy-preserving unique-visitor estimate.
 	// Guarded by metricsMu; never persisted (see metrics.go).