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.

docker-compose.selfhost.yml

@@ -53,6 +53,11 @@ services:
       # Downloaded media. Defaults to a Docker-managed named volume; set DOWNLOAD_HOST_PATH in .env
       # to a host directory (e.g. one your Plex server reads) to keep the Plex-style tree there.
       - ${DOWNLOAD_HOST_PATH:-siftlode_downloads}:/downloads
+      # Plex library, read-only. The Plex module streams files itself, so it needs to SEE them: set
+      # PLEX_MEDIA_HOST_PATH in .env to the host directory holding the library (the local side of the
+      # admin "path map", whose default is /data=/plex-media). Left unset it falls back to an empty
+      # named volume — harmless, and the Plex module is off by default.
+      - ${PLEX_MEDIA_HOST_PATH:-siftlode_plex_media}:/plex-media:ro
     security_opt:
       - no-new-privileges:true
     cap_drop:
@@ -74,6 +79,9 @@ services:
   worker:
     image: git.b1fr0st.eu/peter/siftlode:${IMAGE_TAG:-latest}
     command: ["python", "-m", "app.worker"]
+    # tini as PID 1: reaps orphaned ffmpeg/yt-dlp grandchildren when the watchdog hard-kills a
+    # download child's process group (the worker reaps its own direct children via subprocess.wait).
+    init: true
     env_file:
       - .env
     environment:
@@ -93,6 +101,8 @@ services:
     networks: [internal]
     volumes:
       - ${DOWNLOAD_HOST_PATH:-siftlode_downloads}:/downloads
+      # Same read-only Plex library as the API — the worker resolves library paths too.
+      - ${PLEX_MEDIA_HOST_PATH:-siftlode_plex_media}:/plex-media:ro
     security_opt:
       - no-new-privileges:true
     cap_drop:
@@ -114,6 +124,8 @@ services:
 volumes:
   siftlode_pgdata:
   siftlode_downloads:
+  # Empty fallback for the Plex media mount when PLEX_MEDIA_HOST_PATH is unset (Plex module off).
+  siftlode_plex_media:
 
 networks:
   internal: