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/auth.py

@@ -7,12 +7,18 @@ from sqlalchemy import select
 from app.db import SessionLocal
 from app.models import User
 from app.security.oidc import oauth
+from app.services import diagnostics
 
 router = APIRouter(prefix="/auth", tags=["auth"])
 
 
 @router.get("/login")
 async def login(request: Request):
+    # If OIDC isn't configured, the client was never registered and calling it
+    # would raise an opaque 500. Send the operator to the setup check instead,
+    # which spells out exactly which variables are missing.
+    if not diagnostics.oidc_configured():
+        return RedirectResponse(url="/setup")
     redirect_uri = request.url_for("auth_callback")
     return await oauth.pocketid.authorize_redirect(request, redirect_uri)