rebrand: MangaBM → BookmarkManager, add novel library support (#15)

Two intertwined changes — the rebrand and the novel library were developed on
the same branch because the novel UI plumbing is part of the new "Bookmark
Manager" wordmark in the web shell.

## What it does

- **Rebrand**: MangaBM → BookmarkManager across the Go module, compose stack,
  env vars, Traefik hostnames, container/image names, userscript storage
  prefixes (`mangabm:cache` → `bmgr:manga:cache`, `mangabm:queue` → `bmgr:manga:queue`),
  and docs.
- **Novel library**: same backend, two libraries. New `kind` column splits
  bookmarks into `manga` / `novel`; PUT validates it. Two userscripts:
  - `manga-bookmark.user.js` — unchanged behaviour, just stamps its own `kind`.
  - `novel-bookmark.user.js` — separate Violentmonkey install with adapters
    for **novelfull.com** (polled via headless browser — Cloudflare JS
    challenge) and **lightnovelworld.net** (polled via plain TLS).
- **Web UI**: library switch on the app shell. Login art, libswitch, and
  novel-site colours from the Cinder design snapshot.

## Plumbing

- `addedColumns` ALTER for `kind` runs on first start after upgrade; every
  pre-existing row is backfilled to `'manga'`. No manual SQL, no down-time.
- `ALLOWED_ORIGINS` gains the two novel sites.
- New `NOVEL_USERSCRIPT_PATH` env (default `/userscript/novel-bookmark.user.js`),
  bindmounted alongside the manga script.
- Traefik router names `mangabm*` → `bmapi*` / `bmweb*`.

## Test status

- `go test ./...` — green
- `node --test userscript/test/logic.test.js` — 34 pass
- `node --test userscript/test/novel-logic.test.js` — 11 pass
- `node --check` on both userscripts — clean

## Notes for the redeploy

.env keys were renamed (`MANGA_API_HOST` → `BOOKMARK_API_HOST`,
`MANGA_WEB_HOST` → `BOOKMARK_WEB_HOST`). Update DNS / Traefik labels on the
prod override before pulling, otherwise the public hostnames go dark.
See the redeploy instructions I'll post next to this PR.

Reviewed-on: #15
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/api/handlers.go

@@ -7,7 +7,7 @@ import (
 	"strings"
 	"time"
 
-	"mangabm/backend/internal/store"
+	"bookmarkmanager/backend/internal/store"
 )
 
 // Handler serves the userscript-facing JSON bookmark API.
@@ -78,6 +78,15 @@ func (h *Handler) Put(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Same rule as status: empty means "keep the stored value". An unknown
+	// value is a client bug, not something to silently coerce to manga.
+	switch b.Kind {
+	case "", store.KindManga, store.KindNovel:
+	default:
+		writeJSON(w, http.StatusBadRequest, map[string]string{"error": "invalid kind"})
+		return
+	}
+
 	// Candidate timestamp, not a decision: Upsert keeps the stored one unless
 	// reading progress actually moved. Any client value is ignored.
 	b.UpdatedAt = time.Now().UnixMilli()