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/storage_test.go

@@ -64,6 +64,9 @@ func TestInsertStoryAndGUIDSeen(t *testing.T) {
 func TestGetStoryReaderText(t *testing.T) {
 	setupTestDB(t)
 
+	// Reader text is only served for classified, non-sentinel stories — the same
+	// filter the public /api/article endpoint applies so discarded/unclassified
+	// bodies can't be pulled by id enumeration.
 	s := &Story{
 		GUID:       "reader-guid",
 		Headline:   "Reader Headline",
@@ -71,6 +74,8 @@ func TestGetStoryReaderText(t *testing.T) {
 		Content:    "First paragraph.\n\nSecond paragraph.",
 		ArticleURL: "https://example.com/reader",
 		Source:     "Src",
+		Channel:    "tech",
+		Classified: true,
 		SeenAt:     1700000000,
 	}
 	if err := InsertStory(s); err != nil {
@@ -97,7 +102,7 @@ func TestGetStoryReaderText(t *testing.T) {
 
 	// A story ingested before content capture (no content) is still found, with
 	// an empty content string so the reader falls back to the lede.
-	bare := &Story{GUID: "bare-guid", Headline: "H", Lede: "Only a lede.", ArticleURL: "https://a.com", Source: "S", SeenAt: 1}
+	bare := &Story{GUID: "bare-guid", Headline: "H", Lede: "Only a lede.", ArticleURL: "https://a.com", Source: "S", Channel: "tech", Classified: true, SeenAt: 1}
 	if err := InsertStory(bare); err != nil {
 		t.Fatal(err)
 	}
@@ -120,6 +125,19 @@ func TestGetStoryReaderText(t *testing.T) {
 	if _, _, found, err = GetStoryReaderText(999999); err != nil || found {
 		t.Errorf("unknown id: found=%v err=%v, want found=false err=nil", found, err)
 	}
+
+	// A discarded story must not be readable via /api/article, even by direct id.
+	discarded := &Story{GUID: "disc-guid", Headline: "H", Lede: "hidden", Content: "secret body", ArticleURL: "https://d.com", Source: "S", Channel: "_discarded", Classified: true, SeenAt: 1}
+	if err := InsertStory(discarded); err != nil {
+		t.Fatal(err)
+	}
+	var discID int64
+	if err := Get().QueryRow(`SELECT id FROM stories WHERE guid = ?`, discarded.GUID).Scan(&discID); err != nil {
+		t.Fatal(err)
+	}
+	if _, _, found, err = GetStoryReaderText(discID); err != nil || found {
+		t.Errorf("discarded story: found=%v err=%v, want found=false", found, err)
+	}
 }
 
 func TestInsertStoryDuplicateGUID(t *testing.T) {