share: dockerize the file-share pipeline and document it

The Python half of the short-lived share links lived in the repo; the Apache
config and the cron entries that make it work were hand-placed on the host, so
the feature could not be rebuilt from a checkout. This adds the missing half.

Scripts (defaults unchanged, so existing bare-metal cron keeps working):
* scan_shares.py / revoke_shares.py take their paths from CONJURER_SHARE_*
  instead of hardcoding the Pi layout, and create their parent dirs;
* the revoke TTL is now CONJURER_SHARE_TTL_SECONDS. Its --help claimed "2min"
  while the code used a hardcoded 3600 - the help text now reports the real,
  configured value.

New share service:
* docker/Dockerfile.share - Apache + the two jobs, reusing the same scripts
  rather than forking copies;
* docker/share-vhost.conf.tpl - the previously undocumented Apache half.
  Three settings are load-bearing and commented as such: +FollowSymLinks (the
  shares ARE symlinks), -Indexes (a listing would expose every live token), and
  a deny rule for dotfiles (revoke_shares.py keeps .downloads.json - a map of
  every live token - inside the served directory);
* docker/entrypoint.share.sh - renders the vhost, seeds the index on first run,
  then runs scanner/revoker in sleep loops beside Apache (no cron, so their
  output shows up in docker logs);
* compose + env example, incl. the two volumes that MUST be shared with the
  musician (it creates the links and reads the index).

docs/deployment/FILE_SHARING.md documents the mechanism, both deployment routes
(docker and existing host Apache), the Discord command, why each Apache setting
matters, verification commands, and the known limitations - notably that a link
nobody ever downloads is never revoked, since the TTL starts at first download.

Verified: scanner and revoker exercised end-to-end against temp dirs (index
built; token recorded from a combined-format log line and the symlink unlinked).
Compose file not validated - no docker on this machine.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker/env/musician.env.example

@@ -15,3 +15,17 @@ CONJURER_MUSIC_FOLDER=/music
 # Runtime paths (mounted volume) — playlists/logs the service writes.
 CONJURER_MUSICIAN_BASE=/data
 CONJURER_LOGSTORE=/data/logs
+
+# --- File sharing (optional; see docs/deployment/FILE_SHARING.md) --------
+# The musician SEARCHES the index and CREATES the symlinks; the share container
+# builds the index, serves the links and revokes them. Both must therefore point
+# at the same two host paths — mount them into this container as well:
+#   - /srv/share/db:/srv/share/db
+#   - /srv/share/links:/var/www/html/share
+# Leave these unset to keep the feature off (the endpoints then just find
+# nothing).
+# CONJURER_SHARE_DB=/srv/share/db/share_scan.json
+# CONJURER_SHARE_DIR=/var/www/html/share
+# Public URL prefix handed back to Discord; host half must match the share
+# container's CONJURER_SHARE_SERVER_NAME.
+# CONJURER_SHARE_BASE_URL=https://czernobog.pl/share