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>

app/routers/dashboard.py

@@ -8,7 +8,7 @@ from sqlalchemy import select
 from app.db import get_db
 from app.models import JobRun
 from app.security.deps import require_auth
-from app.services import beets_service, credential_service, network_service, playlist_service, scheduler_service
+from app.services import beets_service, credential_service, diagnostics, network_service, playlist_service, scheduler_service
 
 router = APIRouter(tags=["dashboard"])
 templates = Jinja2Templates(directory="app/templates")
@@ -34,11 +34,13 @@ async def dashboard(request: Request, user: dict = Depends(require_auth), db=Dep
     )
     recent_tracks = beets_service.recently_added(time.time() - _RECENT_TRACKS_WINDOW_SECONDS)
     public_ip = await asyncio.to_thread(network_service.public_ip)
+    setup_warnings = [c for c in diagnostics.checks() if not c["ok"] and (c["critical"] or c.get("warn"))]
     return templates.TemplateResponse(
         request,
         "dashboard.html",
         {
             "user": user,
+            "setup_warnings": setup_warnings,
             "stats": stats,
             "total_size_human": _human_bytes(stats.get("total_bytes", 0)),
             "playlist_count": len(playlists),