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/reference.md

@@ -37,6 +37,7 @@ Individual service reference cards with URLs and configuration details.
 | [[zot]] | Container registry | indri |
 | [[devpi]] | PyPI caching proxy | k8s |
 | [[cv]] | Resume / CV site | k8s |
+| [[dex]] | OIDC identity provider | k8s (ringtail) |
 | [[docs]] | Documentation site (Quartz) | k8s |
 | [[flyio-proxy]] | Public reverse proxy (Fly.io + Tailscale) | Fly.io |
 | [[automounter]] | SMB share automounter | indri |