fix: align secret key references — backend was looking for DB_USER which doesn't exist

Root cause: backend deployment.yaml referenced secretKeyRef key: DB_USER and
key: DB_PASSWORD, but the live secret only has MYSQL_USER and MYSQL_PASSWORD.
kubectl apply reported secret/mysql-secret as "unchanged" (last-applied matched
desired) so the drift was never caught — new pods got CreateContainerConfigError.

Changes:
- backend/deployment.yaml: DB_USER → key: MYSQL_USER, DB_PASSWORD → key: MYSQL_PASSWORD
- mysql/deployment.yaml: add MYSQL_USER/MYSQL_PASSWORD env vars so the app user
  (scrumapp) is created if MySQL ever reinitializes from a fresh PVC
- mysql/secret.yaml: remove stale commented-out block with old key names

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

k8s/base/mysql/secret.yaml

@@ -12,20 +12,8 @@ data:
   MYSQL_PASSWORD: c2NydW1wYXNz
   DB_NAME: c2NydW1fbWFuYWdlcg==
 
-# apiVersion: v1
-# kind: Secret
-# metadata:
-#   name: mysql-secret
-#   labels:
-#     app.kubernetes.io/name: mysql
-#     app.kubernetes.io/component: database
-# type: Opaque
-# data:
-#   # Base64 encoded values — change these for production!
-#   # echo -n 'scrumpass' | base64 => c2NydW1wYXNz
-#   # echo -n 'root' | base64 => cm9vdA==
-#   # echo -n 'scrum_manager' | base64 => c2NydW1fbWFuYWdlcg==
-#   MYSQL_ROOT_PASSWORD: c2NydW1wYXNz
-#   DB_USER: cm9vdA==
-#   DB_PASSWORD: c2NydW1wYXNz
-#   DB_NAME: c2NydW1fbWFuYWdlcg==
+# Decode reference:
+#   MYSQL_ROOT_PASSWORD: scrumpass
+#   MYSQL_USER:          scrumapp
+#   MYSQL_PASSWORD:      scrumpass
+#   DB_NAME:             scrum_manager