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/deployment.yaml

@@ -44,6 +44,18 @@ spec:
             # Allow root to connect from backend pods (any host), not just localhost.
             - name: MYSQL_ROOT_HOST
               value: "%"
+            # Create the app user on first init. Required if PVC is ever wiped and
+            # MySQL reinitializes — otherwise scrumapp user won't exist and backend fails.
+            - name: MYSQL_USER
+              valueFrom:
+                secretKeyRef:
+                  name: mysql-secret
+                  key: MYSQL_USER
+            - name: MYSQL_PASSWORD
+              valueFrom:
+                secretKeyRef:
+                  name: mysql-secret
+                  key: MYSQL_PASSWORD
           volumeMounts:
             - name: mysql-data
               mountPath: /var/lib/mysql