feat: RSA key management for ActivityPub signing

- keys.py: Generate/load RSA-2048 keypairs, sign activities
- setup_keys.py: CLI to generate keys
- Real RsaSignature2017 signing (falls back to placeholder if no keys)
- Public key included in actor profile
- Private keys gitignored

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

requirements.txt

@@ -1,3 +1,4 @@
 fastapi>=0.109.0
 uvicorn>=0.27.0
 requests>=2.31.0
+cryptography>=42.0.0