Generic CDN for any storage backend

The generic CDN mode is a 302 to {base URL}/{object key}. It was wired to S3 only (STORAGE_MODE=s3 plus S3_CDN_PREFIX) even though it works with any public bucket behind a CDN.

• New CDN_BASE_URL variable, available on the s3 and gcs storage modes. S3_CDN_PREFIX stays as a deprecated alias with a one-time warning at boot.
• An explicitly configured base URL wins over the implicit GCS direct mode, which used to shadow it on GCS deployments with signing credentials.
• The settings endpoint reports the resolved CDN_BASE_URL and a CDN type of generic. The dashboard shows it.
• Helm: CDN_BASE_URL sits under the useGenericCDN toggle. Both keys are optional, so secrets holding only the legacy variable keep working.