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/credentials.py

@@ -3,6 +3,7 @@ from fastapi.responses import RedirectResponse
 from fastapi.templating import Jinja2Templates
 
 from app.db import get_db
+from app.security import crypto
 from app.security.deps import require_auth
 from app.services import credential_service
 
@@ -31,6 +32,7 @@ async def credentials_index(request: Request, user: dict = Depends(require_auth)
             "core_scopes": credential_service.CORE_SCOPES,
             "configured": configured,
             "enabled": enabled,
+            "key_present": crypto.key_present(),
         },
     )
 
@@ -47,7 +49,13 @@ async def save_credentials(
         for field, value in form.items()
         if field in credential_service.SCOPE_FIELDS[scope] and value != ""
     }
-    credential_service.set_credentials(db, scope, values)
+    try:
+        credential_service.set_credentials(db, scope, values)
+    except RuntimeError:
+        # Almost always the encryption key is missing, so the value can't be
+        # encrypted. Redirect back; the page shows a key-missing banner with
+        # exactly what to do.
+        return RedirectResponse(url="/settings/credentials", status_code=303)
     return RedirectResponse(url="/settings/credentials", status_code=303)