fix: keep MANGA_WEB_HOST unset in .env.example and make no-op saves inert

Re-review of the fix wave found the .env.example edit defeated the fix
it belonged to: shipping MANGA_WEB_HOST=manga.example.com re-supplied
the value that ${MANGA_WEB_HOST:?} exists to reject, so a fresh
`cp .env.example .env` started fine and Traefik published the UI router
on a domain the operator does not own. Left commented, matching
MANGA_API_HOST; DEPLOY.md 1 now lists it among the required variables.

Also:
- uiChapter leaves last_chapter too, not only last_chapter_url, when the
  submitted number is unchanged. It used to rewrite the display string
  ("45.0" to "45") behind a frozen updated_at.
- Design spec 4.2 documents the two-secret key derivation.
- Corrected the pruneLocked aliasing rationale and the stale
  sessionKeyPurpose comment.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/session.go

@@ -18,7 +18,8 @@ const (
 	// 60 days: long enough that a phone stays logged in between reading spells.
 	sessionTTL = 60 * 24 * time.Hour
 	// Domain separation, so the session key can never collide with any other
-	// use of API_TOKEN. Changing this string logs everyone out.
+	// use of the secrets it is derived from. Changing this string logs
+	// everyone out.
 	sessionKeyPurpose = "mangabm-web-session-v1"
 )
 
@@ -169,9 +170,9 @@ func (l *loginLimiter) reset(ip string) {
 func (l *loginLimiter) pruneLocked(ip string, now time.Time) []time.Time {
 	cutoff := now.Add(-loginWindow)
 	// In-place filter: kept reuses the backing array of the slice being
-	// ranged over. The range expression captures the slice header once at
-	// the start, so the append cursor (kept) can never outrun the read
-	// cursor (the range index) — safe to alias.
+	// ranged over. Safe to alias because append writes at index len(kept),
+	// which is always <= the range index i, and element i is read before
+	// that write — the write cursor can never overtake the read cursor.
 	kept := l.failures[ip][:0]
 	for _, at := range l.failures[ip] {
 		if at.After(cutoff) {

backend/web.go

@@ -254,12 +254,15 @@ func (h *webHandler) uiFavorite(w http.ResponseWriter, r *http.Request) {
 
 // uiChapter forces the read chapter to a value the user typed.
 //
-// When that value actually changes the number, it also clears last_chapter_url:
-// that URL points at the chapter actually read, and once the number is forced
-// elsewhere it would send the reader backwards. ContinueURL then falls back to
-// the series page, which is always right. Resubmitting the same number — the
-// form is pre-filled, so a bare tap of Save is an easy accidental submit —
-// leaves last_chapter_url untouched instead of destroying it for no reason.
+// Writing the number also clears last_chapter_url: that URL points at the
+// chapter actually read, and once the number is forced elsewhere it would send
+// the reader backwards. ContinueURL then falls back to the series page, which
+// is always right.
+//
+// A submit that does not change the number touches nothing. The form is
+// pre-filled, so a bare tap of Save is an easy accidental submit; it must not
+// destroy last_chapter_url, nor rewrite the last_chapter display string ("45.0"
+// to "45") behind a frozen updated_at.
 func (h *webHandler) uiChapter(w http.ResponseWriter, r *http.Request) {
 	b, ok := h.loadForMutation(w, r)
 	if !ok {
@@ -278,9 +281,9 @@ func (h *webHandler) uiChapter(w http.ResponseWriter, r *http.Request) {
 
 	if num != b.LastChapterNum {
 		b.LastChapterURL = ""
+		b.LastChapter = raw
+		b.LastChapterNum = num
 	}
-	b.LastChapter = raw
-	b.LastChapterNum = num
 	b.UpdatedAt = time.Now().UnixMilli()
 	h.saveAndRenderCard(w, b)
 }

backend/web_test.go

@@ -357,14 +357,16 @@ func TestChapterOverrideNoOpPreservesURLAndUpdatedAt(t *testing.T) {
 	srv, store := newWebTestServer(t, cfg)
 	before := seed(t, store, Bookmark{
 		Key: "asura:solo", Site: "asura", SeriesID: "solo",
-		Title: "Solo Leveling", LastChapter: "45", LastChapterNum: 45,
+		Title: "Solo Leveling", LastChapter: "45.0", LastChapterNum: 45,
 		LastChapterURL: "https://example.test/ch/45", SeriesURL: "https://example.test/solo",
 		UpdatedAt: 1_000_000,
 	})
 
 	// The chapter form is pre-filled with the current value, so tapping Save
-	// without editing resubmits the unchanged number. That must be a no-op:
-	// it must not silently clear last_chapter_url or move updated_at.
+	// without editing resubmits the unchanged number. That must be a no-op: it
+	// must not clear last_chapter_url, rewrite the last_chapter display string,
+	// or move updated_at. The seed stores "45.0" against 45 so the display
+	// string differs from what the form submits back.
 	rr := httptest.NewRecorder()
 	srv.ServeHTTP(rr, uiRequest(t, cfg, http.MethodPost,
 		"/ui/bookmarks/asura:solo/chapter", url.Values{"chapter": {"45"}}))
@@ -380,6 +382,10 @@ func TestChapterOverrideNoOpPreservesURLAndUpdatedAt(t *testing.T) {
 		t.Fatalf("LastChapterURL = %q, want preserved %q on a no-op save",
 			after.LastChapterURL, before.LastChapterURL)
 	}
+	if after.LastChapter != before.LastChapter {
+		t.Fatalf("LastChapter = %q, want preserved %q on a no-op save",
+			after.LastChapter, before.LastChapter)
+	}
 	if after.UpdatedAt != before.UpdatedAt {
 		t.Fatalf("UpdatedAt = %d, want unchanged %d on a no-op save",
 			after.UpdatedAt, before.UpdatedAt)