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/static/js/reader.js

@@ -143,10 +143,20 @@
           serverRead[id] = 1; readSet[id] = 1; paintCard(id, true);
         });
         (data.bookmarked || []).forEach(function (id) { setBookmarkQuiet(id, true); });
-        // Push up reads made on this device before the account knew them.
-        ids.forEach(function (id) {
-          if (readSet[id] && !serverRead[id]) postState("/api/read", { id: Number(id), read: true });
-        });
+        // Migrate device-local reads the account doesn't have yet — but only
+        // once per device. After the first sync the server is authoritative, so
+        // a story the user later marks unread on another device stays unread
+        // instead of being perpetually resurrected from this device's stale
+        // local set on every page load.
+        var MIGRATED_KEY = "pete.readMigrated.v1";
+        var migrated = false;
+        try { migrated = localStorage.getItem(MIGRATED_KEY) === "1"; } catch (e) {}
+        if (!migrated) {
+          ids.forEach(function (id) {
+            if (readSet[id] && !serverRead[id]) postState("/api/read", { id: Number(id), read: true });
+          });
+          try { localStorage.setItem(MIGRATED_KEY, "1"); } catch (e) {}
+        }
         saveRead(readSet);
       })
       .catch(function () {});