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) {