P2 minor set: password off argv, configurable share gate, POST logout, log path check

- Navidrome password is now passed to curl via stdin (--data-urlencode "p@-")
  in navidrome-scan.sh and pipeline-status.sh, so it no longer appears in
  ps/proc. Verified the query sent is identical and a live scan still triggers.
- MIN_ARTIST_DIRS (the share-health gate) is now a setting, threaded through to
  the pipeline env, so a user with a small library can lower it instead of the
  scan/sync being permanently blocked by the hardcoded 500.
- /auth/logout is now POST-only (with a nav form + aria-label), so a drive-by
  GET can't log the user out; enforced allowed_email already landed separately.
- view_log now confirms the run's log_path resolves under the logs dir before
  serving it (defense in depth).

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

pipeline/lib/sync-bandcamp.sh

@@ -41,7 +41,7 @@ fi
 # equivalent (same pattern as navidrome-scan.sh's share-health gate).
 LIB="${MUSIC_DATA_DIR:-/data/music}/Library"
 CANARY="$LIB/.navidrome-canary"
-MIN_ARTIST_DIRS=500
+MIN_ARTIST_DIRS="${MIN_ARTIST_DIRS:-500}"
 if [[ ! -r "$CANARY" ]]; then
   log "ERROR: canary $CANARY missing/unreadable — share looks unhealthy — aborting sync"
   exit 1