BUG: Race condition in attachment info building

[andrinoff]

Describe the bug
Attachment processing in daemon builds info slice without synchronization. Concurrent access from multiple clients causes data race.

To reproduce

1. Build with -race flag
2. Multiple clients request same email attachments
3. Race detector reports concurrent slice access

Expected behavior
Use mutex or return copy of attachment info

Matcha version
master

OS
All

Additional context
File: daemon/handler.go lines 103-115
Data race under concurrent load