Deploy Dex OIDC identity provider with Grafana SSO (#222)

## Summary
- Deploys Dex OIDC identity provider on ringtail k3s cluster as central authentication service
- Integrates Grafana as first SSO client via `auth.generic_oauth`
- Uses Kubernetes CRD storage backend (no PVC needed)
- All secrets (bcrypt hash, client secrets) injected via ExternalSecrets from 1Password item "Dex (blumeops)"
- NixOS-built container image via `containers/dex/default.nix`

## Pre-requisites (manual, before deployment)
1. Create 1Password item "Dex (blumeops)" in `blumeops` vault with fields:
   - `password`: strong generated password for Dex login
   - `static-password-hash`: bcrypt hash of above (`htpasswd -BnC 10 eblume`, copy hash after `eblume:`)
   - `grafana-client-secret`: random 32-char hex (`openssl rand -hex 16`)
2. Build container: `mise run container-tag-and-release dex v1.0.0`

## Deployment sequence
1. Build container: `mise run container-tag-and-release dex v1.0.0`
2. Deploy Caddy: `mise run provision-indri -- --tags caddy`
3. Sync ArgoCD: `argocd app sync apps` → `argocd app sync dex`
4. Verify Dex: `curl https://dex.ops.eblu.me/.well-known/openid-configuration`
5. Sync Grafana: `argocd app sync grafana-config` → `argocd app sync grafana`
6. Test SSO: Visit `https://grafana.ops.eblu.me/login`, click "Sign in with Dex"

## Verification
- [ ] Container image exists: `mise run container-list` shows `dex:v1.0.0-nix`
- [ ] `curl https://dex.ops.eblu.me/.well-known/openid-configuration` returns valid OIDC discovery
- [ ] `curl https://dex.ops.eblu.me/healthz` returns healthy
- [ ] Grafana login shows "Sign in with Dex" button alongside local login
- [ ] OIDC flow: click Dex → enter credentials → redirect back → logged in as Admin
- [ ] Break-glass: local admin login still works
- [ ] `mise run services-check` passes

## Files changed
| File | Action | Purpose |
|------|--------|---------|
| `containers/dex/default.nix` | Create | NixOS container build |
| `argocd/apps/dex.yaml` | Create | ArgoCD app targeting ringtail |
| `argocd/manifests/dex/*` (8 files) | Create | K8s manifests (RBAC, ExternalSecret, Deployment, Service, Ingress) |
| `argocd/manifests/grafana-config/external-secret-dex-oauth.yaml` | Create | Grafana OIDC client secret |
| `argocd/manifests/grafana-config/kustomization.yaml` | Modify | Add new ExternalSecret resource |
| `argocd/manifests/grafana/values.yaml` | Modify | Add `auth.generic_oauth` config + envFromSecrets |
| `ansible/roles/caddy/defaults/main.yml` | Modify | Add `dex.ops.eblu.me` reverse proxy entry |
| `docs/changelog.d/feature-dex-oidc.feature.md` | Create | Changelog fragment |

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/222

docs/changelog.d/feature-dex-oidc.feature.md

@@ -0,0 +1 @@
+Deploy Dex OIDC identity provider on ringtail with Grafana as first SSO client.