Add share_file tool and session-lifetime file storage ...

Session file directories now live until the session is deleted, not
24h TTL. Cleanup loop only removes orphaned dirs (session gone from DB).

New share_file tool: copies any file to the session directory and returns
a clickable download URL. Navi can call this after generating any file
the user will want to keep.

New GET /sessions/{id}/files/{filename} endpoint serves files with
correct Content-Disposition (inline for images/HTML/PDF, attachment
for everything else).

Added PUBLIC_URL config key for building correct download links behind
reverse proxies.

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