feat(keycloak): provision SSO client secrets + re-enable oauth2-proxy (R-IDP-01/02/04)

Generate the 3 OIDC client secrets (SOPS-encrypted, minimal manifests) so the realm
import can inject the grafana + oauth2-proxy client secrets and both consumers match:
- platform/keycloak/keycloak-realm-clients.sops.yaml (grafana + oauth2-proxy client secrets)
- platform/oauth2-proxy/oauth2-proxy-credentials.sops.yaml (client-id/secret/cookie)
- platform/monitoring/grafana-oidc-credentials.sops.yaml (grafana client secret)
Wire each into its component kustomization and re-enable oauth2-proxy in platform +
platform-config (reverses the temp deferral now that its secret exists).

Values generated by PM per operator instruction (random; recorded out-of-repo at
~/.config/home-k3s/keycloak-sso-secrets.txt for later rotation). Encrypted via sops in the
dev container to the age recipient in .sops.yaml; plaintext never committed.

Serves: R-IDP-01, R-IDP-02, R-IDP-04, R-SEC-01.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

platform/keycloak/keycloak-realm-clients.sops.yaml

@@ -0,0 +1,29 @@
+apiVersion: v1
+kind: Secret
+metadata:
+    name: keycloak-realm-clients
+    namespace: keycloak
+type: Opaque
+stringData:
+    grafana-client-secret: ENC[AES256_GCM,data:J6grYrkJVubGNS+z1WrcUuvjKu2Z/fETWNXsKkjQnk/RlitaMjDqkazglKgy8PkEymNLNuVao48gksnuNLQ6aA==,iv:9sw5IdW+7GuwsRjn4NSi2JVZh9mTE2hiWJQYQS7w2bA=,tag:2hOUFeFqcQJsPeC8e6U4RA==,type:str]
+    oauth2-proxy-client-secret: ENC[AES256_GCM,data:hE9mN6A6uTL3GpqOCGb3T0qVR9bcY4wcqNlgcrs9+uezSeci/Vs9uiXeFzfD57klHe8nEqxF1bCftD0GVFadhQ==,iv:rP2CbbGG4LWmGyV0adrlsmn2GtQLH8hSlqbioOAD824=,tag:p28vfuSZxYMs7XnAjslBWg==,type:str]
+sops:
+    kms: []
+    gcp_kms: []
+    azure_kv: []
+    hc_vault: []
+    age:
+        - recipient: age175l8mdpdfu7hq0aml3exs3gykarwjmuvr23sq56s3wjejnyk44wsp4mfzg
+          enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSAvaU5Ba1Ztb2JydVQ5Q1cr
+            V3lOV2RwNVNjUFJ3MmRocEYyb0JibWRyTHpzCk5wNGRaNXVhcGNNS3RoN0VWRUdz
+            VHFtbS95c0RQajhUTmJPazdUTTRqZ3MKLS0tIHh0cXJsQ1k3azVHMWdZcWpQTjhP
+            YXJGUUJiTnFndFhDZDU1bmxXTENCbmsKuhb3RQ3LZmuTPAXEck8b6iuZ5vxLb9fV
+            +fQELmU4kOTbZrPSxy+kpttLG5Cy5d6X2ieZXBf6opzpLTUhlBOkHA==
+            -----END AGE ENCRYPTED FILE-----
+    lastmodified: "2026-08-25T03:06:30Z"
+    mac: ENC[AES256_GCM,data:wJ6Mf7yFXnnnuo/IhfGdDYcgNUzzNv2bFGN6wPwmVRr1To4rt3DGb1JZO19OxvloWYrGAyJaeXzC5NCLGDZOzS99TBmMy1TIeEYUqd6zEmFofEla0mZyLFGLsU/JUIVL3d1l4ECPmcHpUnEjJf4o4kE8BxkWAyHSMWktB69a1cc=,iv:7E7T7cFpAcT42ryTfqbDUoWr0d23a9rCI3n//LgBFS8=,tag:zVXOh/hjQNousg85TZw/Zg==,type:str]
+    pgp: []
+    encrypted_regex: ^(data|stringData)$
+    version: 3.9.4

platform/keycloak/kustomization.yaml

@@ -26,5 +26,7 @@ resources:
   - https://raw.githubusercontent.com/keycloak/keycloak-k8s-resources/26.0.6/kubernetes/kubernetes.yml
   # --- Admin (break-glass) credential — SOPS-encrypted, referenced by name (ADR-0007, R-IDP-04) ---
   - keycloak-admin-credentials.sops.yaml
+  # --- OIDC client secrets (grafana + oauth2-proxy) the realm import injects via placeholders ---
+  - keycloak-realm-clients.sops.yaml
   # --- Grafana dashboard for Keycloak health (plain ConfigMap; R-OBS-03) ---
   - grafana-dashboard.yaml