Document Dex OIDC and add services-check integration (#223)

## Summary
- Create Dex reference card (`docs/reference/services/dex.md`) with quick reference, architecture, identity source, storage, OIDC clients, secrets, and endpoints
- Write federated login explanation article (`docs/explanation/federated-login.md`) covering the Dex + Forgejo two-layer auth model, login flow, and break-glass access
- Add Dex to `services-check` (HTTP health endpoint + k3s pod check)
- Update Grafana docs with new Authentication section documenting SSO via Dex
- Update Forgejo docs with OAuth2 Provider section documenting its role as upstream identity source
- Add Dex to ringtail workloads table and reference service index
- Move `adopt-oidc-provider` plan to `completed/` with final design reflecting actual implementation

## Test plan
- [ ] `mise run services-check` passes (includes new Dex checks)
- [ ] `docs-check-links` passes (all wiki-links resolve)
- [ ] `docs-check-index` passes (new docs are indexed)

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/223

docs/reference/infrastructure/ringtail.md

@@ -68,6 +68,7 @@ Sync order: `1password-connect-ringtail` -> `external-secrets-crds-ringtail` ->
 | [[frigate]] | `frigate` | NVR with GPU-accelerated detection (RTX 4080) |
 | [[frigate]]-notify | `frigate` | MQTT-to-ntfy alert bridge |
 | Mosquitto | `mqtt` | MQTT broker for Frigate events |
+| [[dex]] | `dex` | OIDC identity provider (Forgejo-backed) |
 | [[ntfy]] | `ntfy` | Push notification server |
 | nvidia-device-plugin | `nvidia-device-plugin` | Exposes GPU to pods via CDI + nvidia RuntimeClass |