Replaces local filesystem document storage with S3-compatible object storage.
Adds embedded SeaweedFS 'weed mini' for local development and Docker deployments.
Updates document upload flow to use presigned URLs with a server fallback proxy.
Refactors auth configuration to use BASE_URL and AUTH_SECRET.
BREAKING CHANGE: Renamed BETTER_AUTH_URL to BASE_URL and BETTER_AUTH_SECRET to AUTH_SECRET. Removed legacy document upload/content endpoints. Requires S3 environment variables (auto-configured for embedded SeaweedFS).
