Security hardening and first-run/portability improvements

Security (P0):
- Remove committed session-secret default; auto-generate and persist a
  random secret to the config volume when SESSION_SECRET is unset
  (prevents forgeable session cookies / auth bypass).
- Validate playlist names to a safe charset and render sldl configs via
  literal Python substitution instead of sed (closes a command-injection
  and path-traversal path through playlist names).
- shlex-quote credential values written to shell-sourced env files, and
  strip newlines from values patched into .conf files.
- Render playlist .conf files 0600; warn at startup if the master key is
  co-located with the config volume; document keeping it separate.

Portability:
- Configurable timezone via TZ (default UTC) instead of hardcoded Edmonton.
- Remove personal defaults (navidrome user "andrew", ephemeral.club URLs).
- Ship generic example seeds; move the cross-album dedup keep-list and the
  legacy playlist import to editable config files; drop the personal
  _upgrade.csv.
- Generic VPN reference in docker-compose.snippet.yml.

First-run experience:
- Redirect to /setup instead of 500 when OIDC is unconfigured; surface a
  missing master key inline; entrypoint exits with an actionable message
  when the config folder is not writable.
- Add unauthenticated /health (JSON) and /setup (checklist) diagnostics.

Docs:
- Write docs/ARCHITECTURE.md and docs/MIGRATION.md (previously referenced
  but missing); expand README with ownership, backups, advanced settings,
  and migration guidance.

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

pipeline/lib/pipeline-status.sh

@@ -28,7 +28,7 @@ NOW_TS=$(date +%s)
 NAVIDROME_ENV="${ALEMBIC_CONFIG_DIR:-/config}/pipeline/navidrome/admin.env"
 [ -f "$NAVIDROME_ENV" ] && source "$NAVIDROME_ENV"
 ND_BASE="${ND_BASE:-http://navidrome:4533}"
-ND_USER="${ND_USER:-andrew}"
+ND_USER="${ND_USER:-}"
 ND_PASS="${ND_PASS:-}"
 
 # Tally counters for the header banner. Body collected first, then prepended.
@@ -366,7 +366,7 @@ except Exception as e:
 
 # ---- Build header banner and prepend ----
 HOSTNAME_SHORT=$(hostname -s)
-DATE_HUMAN=$(TZ=America/Edmonton date '+%a %Y-%m-%d %H:%M %Z')
+DATE_HUMAN=$(TZ="${TZ:-UTC}" date '+%a %Y-%m-%d %H:%M %Z')
 BANNER_TITLE="🎵 Music pipeline · $HOSTNAME_SHORT · $DATE_HUMAN"
 BANNER_TALLY="   $OK OK · $WARN warn · $SKIP skip"
 sed -i "1c\\