feat(selfhost): license the repo, and make Plex actually usable on the public compose files

DX-10: the repo invited people to clone, build and self-host while carrying
no LICENSE at all — which legally means all rights reserved, so none of that
was permitted. Adds the verbatim GNU AGPL-3.0 text, the SPDX id in
package.json and a README section. AGPL rather than MIT deliberately: this is
a server application, so the case worth covering is someone running a
modified copy as a network service.

DX-12: .env.example documented PLEX_MEDIA_HOST_PATH -> /plex-media and the
README advertised the Plex library, but only the localdev and home compose
files carried the mount — on both PUBLIC files the module could read Plex's
catalogue and then fail to open a single file, because it streams the media
itself rather than proxying Plex. Both api and worker now mount it read-only
with an empty named volume as the fallback, and self-hosting.md gains the
section that explains the server URL/token, the path map and the mount
together (it had never mentioned Plex).

DX-15: the worker had no init: true on either public file, though localdev
and home both carry tini with the reason written out — without PID 1 reaping,
a watchdog hard-kill of a download's process group orphans its ffmpeg/yt-dlp
grandchildren.

Verified: 'compose config' resolves on both files, the Plex mount comes out
read_only: true on api and worker, and init lands on worker (not just the
bgutil-pot sidecar). Gate green: vitest 671, pytest 896.

frontend/package.json

@@ -1,6 +1,7 @@
 {
   "name": "siftlode-frontend",
   "private": true,
+  "license": "AGPL-3.0-only",
   "type": "module",
   "scripts": {
     "dev": "vite",