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-agent/src/ssh.rs

@@ -0,0 +1,57 @@
+use std::path::Path;
+
+use crate::error::AgentError;
+
+/// Ensure the SSH key pair exists at the given path, generating it if missing.
+/// Returns the public key contents.
+pub fn ensure_ssh_key(key_path: &str) -> Result<String, AgentError> {
+    let private_path = Path::new(key_path);
+    let public_path = private_path.with_extension("pub");
+
+    if private_path.exists() && public_path.exists() {
+        return std::fs::read_to_string(&public_path).map_err(|e| {
+            AgentError::Config(format!("Failed to read SSH public key: {e}"))
+        });
+    }
+
+    // Create parent directory
+    if let Some(parent) = private_path.parent() {
+        std::fs::create_dir_all(parent)?;
+    }
+
+    // Generate ed25519 key pair using ssh-keygen
+    let output = std::process::Command::new("ssh-keygen")
+        .args([
+            "-t",
+            "ed25519",
+            "-f",
+            key_path,
+            "-N",
+            "", // no passphrase
+            "-C",
+            "compliance-scanner-agent",
+        ])
+        .output()
+        .map_err(|e| AgentError::Config(format!("Failed to run ssh-keygen: {e}")))?;
+
+    if !output.status.success() {
+        let stderr = String::from_utf8_lossy(&output.stderr);
+        return Err(AgentError::Config(format!(
+            "ssh-keygen failed: {stderr}"
+        )));
+    }
+
+    // Set correct permissions
+    #[cfg(unix)]
+    {
+        use std::os::unix::fs::PermissionsExt;
+        std::fs::set_permissions(private_path, std::fs::Permissions::from_mode(0o600))?;
+    }
+
+    let public_key = std::fs::read_to_string(&public_path).map_err(|e| {
+        AgentError::Config(format!("Failed to read generated SSH public key: {e}"))
+    })?;
+
+    tracing::info!("Generated new SSH key pair at {key_path}");
+    Ok(public_key)
+}