Migrates attachment and avatar binary serving from gRPC endpoints to a new dedicated HTTP fileserver package, fixing Safari video playback issues and improving architectural separation.

Key changes:
- Created server/router/fileserver package for all binary file serving
- Removed GetAttachmentBinary and GetUserAvatar gRPC endpoints from proto
- Implemented native HTTP handlers with full range request support
- Added authentication support (session cookies + JWT) to fileserver
- New avatar endpoint supports lookup by user ID or username
- Eliminated duplicate auth constants (imports from api/v1)

HTTP endpoints:
- Attachments: /file/attachments/:uid/:filename (unchanged URL)
- Avatars: /file/users/:identifier/avatar (new URL format)

This fixes Safari video/audio playback by using http.ServeContent() which properly handles HTTP 206 Partial Content responses and range request headers.

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>