chore: ignore the go build artifact, mark two accepted limitations

`go build ./...` in backend/ emits `backend/backend` (the module is
`mangabm/backend`), but .gitignore only listed `backend/server` — the name
the Dockerfile uses for its container-internal build. The real artifact was
untracked and unignored, and came close to being committed twice. Keeps the
`server` entry, since a local `go build -o server` still matches the
Dockerfile's naming.

Also records two deliberate limitations as `ponytail:` comments so they are
greppable rather than living only in prose:

- latest.go: the poller's Store.Get + Store.Upsert is not wrapped in a
  transaction, so a client PUT committing between the two is lost to the
  stale re-read. Already documented in CLAUDE.md; the marker names the
  upgrade path (wrap in a tx) and the trigger (more than one user). Note
  this now costs a status change, not just read progress.
- web.go: a swapped card stays on screen when its new status no longer
  matches the active tab. The alternative is a full list round trip per
  toggle; the card showing its new state is the feedback that matters.

Comments only — no logic change. Tests pass, CGO_ENABLED=0 builds.

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

backend/web.go

@@ -262,6 +262,13 @@ func (h *webHandler) loadForMutation(w http.ResponseWriter, r *http.Request) (Bo
 // saveAndRenderCard upserts and renders the row as stored. Upsert decides
 // whether updated_at moves, so the argument's timestamp is only a candidate and
 // the response must come from the return value.
+//
+// ponytail: the swapped card stays put even when its new status no longer
+// matches the active tab, add an hx-swap-oob list refresh if that reads as a
+// bug rather than as feedback. Archiving from the All tab leaves the card on
+// screen until the next list load. The alternative costs a full list round
+// trip on every toggle, and the card visibly showing its new state is the
+// feedback the user needs.
 func (h *webHandler) saveAndRenderCard(w http.ResponseWriter, b Bookmark) {
 	stored, err := h.store.Upsert(b)
 	if err != nil {