fix: add explicit datasource defaults to application.yml

Without an explicit spring.datasource.url, Spring Boot falls back to
jdbc:postgresql://localhost:5432 when the SPRING_DATASOURCE_URL env var
is missing or not picked up. Default now points to the docker-compose
service name (postgres:5432/cameleer_saas).

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

src/main/resources/application.yml

@@ -1,6 +1,10 @@
 spring:
   application:
     name: cameleer-saas
+  datasource:
+    url: ${SPRING_DATASOURCE_URL:jdbc:postgresql://postgres:5432/cameleer_saas}
+    username: ${SPRING_DATASOURCE_USERNAME:cameleer}
+    password: ${SPRING_DATASOURCE_PASSWORD:cameleer_dev}
   jpa:
     open-in-view: false
     hibernate: