936996f6

By: Michael Lynch <git@mtlynch.io>

Scaffold S3-compatible storage backend (stubbed, not yet implemented)

Wire in the s3 and s3config packages, a MediaSource abstraction in
the handlers package, and mutually exclusive --media-dir/--s3-config
flags in both commands. All s3 implementation functions currently
return "not implemented", so no live S3 connection is required.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>