fix: redact internal configuration and database errors

[cloudcarver]

Summary

• stop embedding merged configuration bytes in YAML decode errors
• remove PostgreSQL DSNs and credentials from startup errors and logs
• return a generic public 500 response for generated counter handler failures
• add canary tests for configuration, DSN, and handler error disclosure

Validation

• go test ./...
• go test -race ./lib/conf ./pkg/zcore/model
• go vet ./...
• make gen

Closes #41
Closes #43
Closes #44