Admin plane: validate Authentik OIDC JWTs on /api/v1/admin/*

The admin plane had no authentication: any caller could reach its
routes once they existed. This adds the plane's ONE reusable FastAPI
dependency (require_admin_jwt in api/auth.py) that verifies an
Authentik OIDC JWT — Bearer extraction, RS256-only signature check
against the provider's JWKS, issuer, audience, required exp, and
enforced nbf — and raises the SAME 401 for every failure so the
response never leaks which check failed.

The JWKS is fetched over httpx once per app (JwksCache, 5-minute TTL,
asyncio-lock-guarded, failed fetches never cached) and tests stub the
fetcher, so the suite proves signature/expiry/audience enforcement
with locally generated RSA key pairs and zero network access.

ApiConfig grows the three required OIDC settings
(NESTQUEST_OIDC_ISSUER/AUDIENCE/JWKS_URL) under the existing fail-fast
discipline: an unconfigured issuer must not silently leave the admin
plane open, so from_env raises rather than inventing a default.

This is authentication only — the verified claims are the dependency's
return value, so the nestquest-admins group check is the next task's
addition, not a rewrite. PyJWT[crypto] joins the dev dependency group
(the API deployable's runtime manifest lands in Feature 17).

Maestro task: e70e6bfb-085a-4781-a3f4-276355bbbd19

pyproject.toml

@@ -17,6 +17,7 @@ dev = [
     "fastapi>=0.115",
     "httpx>=0.28",
     "jinja2>=3",
+    "pyjwt[crypto]>=2.9",
     "pytest>=8.0.0",
     "pytest-asyncio>=0.24",
     "pyyaml>=6",