feat: implement SSH key management

- Add backend API endpoints for SSH key CRUD (POST, GET, DELETE)
- Implement Ed25519 key generation with Fernet-encrypted private keys
- Add frontend SSH keys page with generate, list, and delete functionality
- Include copy-to-clipboard for public keys
- Add responsive CSS styles for key cards
- Register ssh_keys router in main.py
- Add basic auth tests for SSH key endpoints

Quality gates: ruff ✓, mypy ✓, typecheck ✓, lint ✓

openspec/changes/ssh-key-management/proposal.md

@@ -0,0 +1,25 @@
+## Why
+
+SSH keys are required for git operations with external providers. Currently the SSHKey model exists but there's no API or UI to generate, manage, or use SSH keys.
+
+## What Changes
+
+- Add backend API endpoints for SSH key CRUD operations
+- Implement Ed25519 key generation with Fernet encryption for private keys
+- Add frontend page for SSH key management
+- Display public keys with copy functionality
+
+## Capabilities
+
+### New Capabilities
+- `ssh-key-management`: Generate and manage SSH keys for git operations
+
+### Modified Capabilities
+- None
+
+## Impact
+
+- `apps/api/src/api/`: New ssh_keys.py router
+- `apps/api/src/models/ssh_key.py`: May need updates
+- `apps/web/src/`: New SSH keys page
+- `apps/api/src/main.py`: Register new router