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>

app/services/pipeline_runner.py

@@ -42,6 +42,7 @@ def _subprocess_env() -> dict:
     env["MUSIC_DATA_DIR"] = str(settings.music_data_dir)
     env["PIPELINE_DIR"] = str(settings.pipeline_dir)
     env["BEETSDIR"] = str(settings.beets_dir)
+    env["MIN_ARTIST_DIRS"] = str(settings.min_artist_dirs)
     return env