feat: add Keycloak authentication for dashboard and API endpoints (#2)

Dashboard: OAuth2/OIDC login flow with PKCE, session-based auth middleware
protecting all server function endpoints, check-auth server function for
frontend auth state, login page gate in AppShell, user info in sidebar.

Agent API: JWT validation middleware using Keycloak JWKS endpoint,
conditionally enabled when KEYCLOAK_URL and KEYCLOAK_REALM are set.

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

Co-authored-by: Sharang Parnerkar <parnerkarsharang@gmail.com>
Reviewed-on: #2

compliance-agent/src/config.rs

@@ -45,5 +45,7 @@ pub fn load_config() -> Result<AgentConfig, AgentError> {
             .unwrap_or_else(|| "0 0 0 * * *".to_string()),
         git_clone_base_path: env_var_opt("GIT_CLONE_BASE_PATH")
             .unwrap_or_else(|| "/tmp/compliance-scanner/repos".to_string()),
+        keycloak_url: env_var_opt("KEYCLOAK_URL"),
+        keycloak_realm: env_var_opt("KEYCLOAK_REALM"),
     })
 }