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/session/session.go

@@ -14,13 +14,13 @@ import (
 )
 
 const (
-	CookieName = "mangabm_session"
+	CookieName = "bmgr_session"
 	// 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 the secrets it is derived from. Changing this string logs
 	// everyone out.
-	sessionKeyPurpose = "mangabm-web-session-v1"
+	sessionKeyPurpose = "bmgr-web-session-v1"
 )
 
 // Key derives the cookie-signing key from both secrets. Sessions are