fix(api): preserve existing database secrets

[calltelemetry-jason]

Summary

• stop rendering a generated api-db-secret when db.useExistingSecret=true
• keep workload secret references pointed at the operator-provided Kubernetes Secret
• add a Helm unit test proving no generated Secret is emitted and references remain present

Verification

• tests/unit/test-api-existing-secret.sh (passed: 10 references, no generated Secret)
• helm lint helm/charts/api --set db.useExistingSecret=true --set db.existingSecretName=api-db-secret (passed)

Context

This unblocks the reproducible backend DevSpace lane needed for REL-255 without placing database credentials in source. It does not close tenant/appliance acceptance by itself.

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📥 Commits

Reviewing files that changed from the base of the PR and between ce57a3c and 37746a6.

📒 Files selected for processing (2)

• helm/charts/api/templates/db_secret.yaml
• tests/unit/test-api-existing-secret.sh

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