fix: let SPRING_DATASOURCE_URL fully control datasource connection

Explicit spring.datasource.url in YAML takes precedence over the env var,
causing deployed containers to connect to localhost instead of the postgres
service. Now the YAML uses ${SPRING_DATASOURCE_URL:...} so the env var
wins when set. Flyway inherits from the datasource (no separate URL).
Removed CAMELEER_DB_SCHEMA — schema is part of the datasource URL.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

deploy/overlays/feature/kustomization.yaml

@@ -23,8 +23,8 @@ patches:
             containers:
               - name: server
                 env:
-                  - name: CAMELEER_DB_SCHEMA
-                    value: "BRANCH_SCHEMA"
+                  - name: SPRING_DATASOURCE_URL
+                    value: "jdbc:postgresql://postgres.cameleer.svc.cluster.local:5432/cameleer3?currentSchema=BRANCH_SCHEMA"
                   - name: CAMELEER_UI_ORIGIN
                     value: "http://BRANCH_SLUG.cameleer.siegeln.net"
   # UI ConfigMap: branch-specific API URL