Private attachments were accessible without authentication if the UUID
was known. Added AuthMiddleware to the GET /api/v1/files/{id} route,
uploader_id tracking on uploads, and channel-level permission checks
(guild READ_MESSAGES, DM participant, admin bypass) in handleServeFile.
Migration 010 adds uploader_id column to attachments table.
8 new access-control tests covering all authorization paths.
