Make postgres Secret name release-scoped via fullname helper

[Copilot]

Hardcoding the Secret name postgres-credentials causes collisions when multiple Helm releases are deployed to the same namespace. Each release must own its own Secret.

Changes

• postgres-secret.yaml — Secret name now uses {{ include "java-backend.fullname" . }}-postgres-credentials, consistent with how all other chart resources are named
• deployment.yaml — SPRING_DATASOURCE_* env vars are rendered explicitly in the template (not via values.yaml) so the secretKeyRef.name can reference the template-computed name
• values.yaml — Removed the three hardcoded SPRING_DATASOURCE_* secretKeyRef entries; those vars are now owned by the deployment template

# Before
metadata:
  name: postgres-credentials          # static — collides across releases

# After
metadata:
  name: {{ include "java-backend.fullname" . }}-postgres-credentials   # e.g. java-backend-postgres-credentials

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