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/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)
 }