Harden download attachment filenames - #139

Summary

• add a shared Content-Disposition attachment helper that derives a basename and strips quoted-string/control-character hazards
• use the helper for direct database downloads, path downloads, disk proxy downloads, and HTTP proxy downloads
• add regression coverage for unsafe database and disk-proxy filenames so downloads still succeed and no injected header appears

Validation

• bun test tests/routes/files.test.ts tests/routes/file-proxy02.test.ts
• bun test
• bunx tsc --noEmit
• bun run format:check
• bun run build
• git diff --check

/claim #5