docs(env): document S3_PUBLIC_ENDPOINT and the /s3 path-prefix behaviour

Follow-up to 4df4d8c, which added the variable. Keeps .env.example an
accurate configuration contract.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.env.example

@@ -58,7 +58,12 @@ AWS_S3_BUCKET=your-s3-bucket-name
 S3_FOLDER_PREFIX=development  # Root folder for all uploads (e.g., 'development', 'staging', 'production')
 # Custom endpoint for S3-compatible storage (DigitalOcean Spaces, MinIO,
 # Contabo). Leave blank for real AWS S3.
+# If it ends in /s3 (MinIO behind an nginx path prefix), the SDK signs against
+# the host root and browser-facing URLs keep the /s3 prefix.
 S3_ENDPOINT=
+# Public base for browser-facing URLs when it differs from the signing
+# endpoint. Defaults to S3_ENDPOINT.
+S3_PUBLIC_ENDPOINT=
 
 # Email (REQUIRED — boot fails without it)
 # The app POSTs JSON to this endpoint to send all transactional email.