feat: add private repository support with SSH key and HTTPS token auth

- Generate SSH ed25519 key pair on agent startup for cloning private repos via SSH
- Add GET /api/v1/settings/ssh-public-key endpoint to expose deploy key
- Add auth_token and auth_username fields to TrackedRepository model
- Wire git2 credential callbacks for both SSH and HTTPS authentication
- Validate repository access before saving (test-connect on add)
- Update dashboard add form with optional auth section showing deploy key and token fields
- Show error toast if private repo cannot be accessed

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

compliance-core/src/config.rs

@@ -24,6 +24,7 @@ pub struct AgentConfig {
     pub scan_schedule: String,
     pub cve_monitor_schedule: String,
     pub git_clone_base_path: String,
+    pub ssh_key_path: String,
     pub keycloak_url: Option<String>,
     pub keycloak_realm: Option<String>,
 }