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/backend/deployment.yaml

@@ -49,12 +49,12 @@ spec:
               valueFrom:
                 secretKeyRef:
                   name: mysql-secret
-                  key: DB_USER
+                  key: MYSQL_USER
             - name: DB_PASSWORD
               valueFrom:
                 secretKeyRef:
                   name: mysql-secret
-                  key: DB_PASSWORD
+                  key: MYSQL_PASSWORD
             - name: DB_NAME
               valueFrom:
                 secretKeyRef: