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/health.py

@@ -0,0 +1,30 @@
+from fastapi import APIRouter, Request
+from fastapi.responses import JSONResponse
+from fastapi.templating import Jinja2Templates
+
+from app.services import diagnostics
+
+router = APIRouter(tags=["health"])
+templates = Jinja2Templates(directory="app/templates")
+
+
+@router.get("/health")
+async def health():
+    """Unauthenticated JSON health check. 200 when everything critical is
+    configured, 503 when something critical is missing so an uptime monitor
+    notices. The body always lists every check either way."""
+    result = diagnostics.summary()
+    status_code = 200 if result["status"] == "ok" else 503
+    return JSONResponse(result, status_code=status_code)
+
+
+@router.get("/setup")
+async def setup(request: Request):
+    """Unauthenticated, human-readable version of the same checks, so a fresh
+    operator can see what still needs configuring before login works."""
+    result = diagnostics.summary()
+    return templates.TemplateResponse(
+        request,
+        "setup.html",
+        {"status": result["status"], "checks": result["checks"]},
+    )