feat(external): mega.nz via megatools in the runtime image (Phase 4c)

Use `megatools dl` (Debian-native apt package) for mega.nz public links rather
than MEGAcmd — no external MEGA apt repo/key to add, one apt line. Adds
`megatools` to the runtime Dockerfile; the fetcher's mega backend now shells
`megatools dl --path <dir> <url>` (key in the #fragment is preserved by the
extractor). gdown (gdrive) is already a pip dep in the runtime image.

NOTE: build.yml builds the image on main/tags only (not dev), so this Dockerfile
change is verified on the next dev→main merge, not by this dev push. The fetcher
code path is unit-tested via the mocked _run_mega_get seam.

With this, all 5 hosts download end-to-end once a celery download-worker runs.
Refs FC #830 (Phase 4c).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -18,13 +18,16 @@ ENV PYTHONUNBUFFERED=1 \
 
 # System deps: ffmpeg (transcode + thumbnails, FC-2), unar (archives, FC-2),
 # libpq for psycopg, postgresql-client + zstd for FC-5 backup/restore
-# (pg_dump + tar --zstd), image libs.
+# (pg_dump + tar --zstd), image libs, megatools (mega.nz public-link downloads
+# for off-platform file-host links, #830 — `megatools dl`; Debian-native, no
+# external MEGA apt repo needed).
 RUN apt-get update && apt-get install -y --no-install-recommends \
         ffmpeg \
         unar \
         libpq5 \
         postgresql-client \
         zstd \
+        megatools \
         libjpeg62-turbo \
         libwebp7 \
         libpng16-16 \