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/storage/userstate.go

@@ -121,11 +121,15 @@ func ListBookmarks(sub string, limit, offset int) ([]Story, error) {
 	return out, rows.Err()
 }
 
-// CountBookmarks returns how many stories the user has bookmarked.
+// CountBookmarks returns how many bookmarked stories the user would see on the
+// bookmarks page. It applies the same classified filter as ListBookmarks so the
+// "N saved" header can't exceed the number of rendered cards.
 func CountBookmarks(sub string) (int, error) {
 	var n int
 	err := Get().QueryRow(
-		`SELECT COUNT(*) FROM user_story_state WHERE user_sub = ? AND bookmarked_at IS NOT NULL`,
+		`SELECT COUNT(*) FROM user_story_state u
+		 JOIN stories s ON s.id = u.story_id
+		 WHERE u.user_sub = ? AND u.bookmarked_at IS NOT NULL AND s.classified = 1`,
 		sub).Scan(&n)
 	return n, err
 }