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/api/mod.rs

@@ -1,3 +1,4 @@
+pub mod auth_middleware;
 pub mod handlers;
 pub mod routes;
 pub mod server;