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-dashboard/src/infrastructure/keycloak_config.rs

@@ -0,0 +1,56 @@
+use super::error::DashboardError;
+
+/// Keycloak OpenID Connect settings.
+#[derive(Debug)]
+pub struct KeycloakConfig {
+    pub url: String,
+    pub realm: String,
+    pub client_id: String,
+    pub redirect_uri: String,
+    pub app_url: String,
+}
+
+impl KeycloakConfig {
+    pub fn from_env() -> Result<Self, DashboardError> {
+        Ok(Self {
+            url: required_env("KEYCLOAK_URL")?,
+            realm: required_env("KEYCLOAK_REALM")?,
+            client_id: required_env("KEYCLOAK_CLIENT_ID")?,
+            redirect_uri: required_env("REDIRECT_URI")?,
+            app_url: required_env("APP_URL")?,
+        })
+    }
+
+    pub fn auth_endpoint(&self) -> String {
+        format!(
+            "{}/realms/{}/protocol/openid-connect/auth",
+            self.url, self.realm
+        )
+    }
+
+    pub fn token_endpoint(&self) -> String {
+        format!(
+            "{}/realms/{}/protocol/openid-connect/token",
+            self.url, self.realm
+        )
+    }
+
+    pub fn userinfo_endpoint(&self) -> String {
+        format!(
+            "{}/realms/{}/protocol/openid-connect/userinfo",
+            self.url, self.realm
+        )
+    }
+
+    pub fn logout_endpoint(&self) -> String {
+        format!(
+            "{}/realms/{}/protocol/openid-connect/logout",
+            self.url, self.realm
+        )
+    }
+}
+
+fn required_env(name: &str) -> Result<String, DashboardError> {
+    std::env::var(name)
+        .map_err(|_| DashboardError::Config(format!("{name} is required but not set")))
+}