- Add startup banner with NO AUTH warning
- Add --max-decompress-gb CLI flag (default 1 GB)
- Add hard cap for --max-upload-mb at 4096 MB (4 GiB)
- Add max_decompress_gb form field parsing
- Update CLI help text with security model documentation
- Add comprehensive security model docs to serve.rs rustdoc
This implements the security constraints required by the bead:
- No built-in authentication (deploy behind reverse proxy)
- No file-path parameters (multipart upload only)
- Hard caps to prevent integer overflow
- Visible security warnings at startup
Closes: pdftract-4li3d
