feat: add role-based user management, audit log, and session tracking

Introduces a full user management system with three roles (admin, operator,
viewer), an audit log, and per-session login history.

Backend:
- app/internal/audit.py: write_audit() helper → audit_log Firestore collection
- app/internal/auth.py: get_role() helper; require_admin_token accepts both
  legacy admin:true claim and new role:"admin" claim for backward compat
- app/routers/users.py: CRUD under /admin/users — list, create (returns
  one-time invite link), get (with sessions), patch role/nodes/name,
  disable, enable, delete; operator role requires ≥1 owned node
- app/routers/links.py: POST /auth/session records sign-in events to
  user_sessions Firestore collection
- app/routers/admin.py: GET /admin/audit paginated endpoint
- app/main.py: register users router

Frontend:
- AuthProvider: exposes role, isAdmin, isOperator, ownedNodeIds from claims
- Nav: role-gated links — viewers get dashboard/calls/incidents/map/alerts/
  trips; operators add nodes/systems/tokens; admins add admin
- admin/page.tsx: new Users tab (list table, create modal, inline edit panel
  with role/nodes editor, disable/enable/delete, login history) and Audit
  Log tab (paginated, color-coded actions)
- login/page.tsx: calls recordSession() on email and Google sign-in
- nodes, systems, tokens pages: role guards redirect viewers to dashboard
- profile/page.tsx: shows accurate role badge and label
- lib/types.ts: UserRole, UserRecord, UserSession, AuditEntry types
- lib/c2api.ts: user management methods + recordSession

Firestore collections added: user_profiles, audit_log, user_sessions
Firebase custom claims schema: { role, owned_node_ids, admin (legacy) }

drb-frontend/components/AuthProvider.tsx

@@ -3,25 +3,33 @@
 import { createContext, useContext, useEffect, useState } from "react";
 import { onAuthStateChanged, signOut as firebaseSignOut, User } from "firebase/auth";
 import { auth } from "@/lib/firebase";
+import type { UserRole } from "@/lib/types";
 
 interface AuthContextType {
   user: User | null;
   loading: boolean;
+  role: UserRole | null;
   isAdmin: boolean;
+  isOperator: boolean;
+  ownedNodeIds: string[];
   signOut: () => Promise<void>;
 }
 
 const AuthContext = createContext<AuthContextType>({
   user: null,
   loading: true,
+  role: null,
   isAdmin: false,
+  isOperator: false,
+  ownedNodeIds: [],
   signOut: async () => {},
 });
 
 export function AuthProvider({ children }: { children: React.ReactNode }) {
   const [user, setUser] = useState<User | null>(null);
   const [loading, setLoading] = useState(true);
-  const [isAdmin, setIsAdmin] = useState(false);
+  const [role, setRole] = useState<UserRole | null>(null);
+  const [ownedNodeIds, setOwnedNodeIds] = useState<string[]>([]);
 
   useEffect(() => {
     return onAuthStateChanged(auth, async (u) => {
@@ -30,12 +38,25 @@ export function AuthProvider({ children }: { children: React.ReactNode }) {
 
       if (u) {
         document.cookie = "drb_session=1; path=/; SameSite=Strict";
-        // Read custom claims to determine admin status
         const result = await u.getIdTokenResult(true);
-        setIsAdmin(!!result.claims.admin);
+        const claims = result.claims;
+
+        // Derive role: prefer granular "role" claim, fall back to legacy "admin" boolean
+        let effectiveRole: UserRole = "viewer";
+        if (claims.role === "admin" || claims.admin) {
+          effectiveRole = "admin";
+        } else if (claims.role === "operator") {
+          effectiveRole = "operator";
+        } else if (claims.role === "viewer") {
+          effectiveRole = "viewer";
+        }
+
+        setRole(effectiveRole);
+        setOwnedNodeIds((claims.owned_node_ids as string[]) ?? []);
       } else {
         document.cookie = "drb_session=; path=/; max-age=0";
-        setIsAdmin(false);
+        setRole(null);
+        setOwnedNodeIds([]);
       }
     });
   }, []);
@@ -45,8 +66,11 @@ export function AuthProvider({ children }: { children: React.ReactNode }) {
     document.cookie = "drb_session=; path=/; max-age=0";
   }
 
+  const isAdmin = role === "admin";
+  const isOperator = role === "operator";
+
   return (
-    <AuthContext.Provider value={{ user, loading, isAdmin, signOut }}>
+    <AuthContext.Provider value={{ user, loading, role, isAdmin, isOperator, ownedNodeIds, signOut }}>
       {children}
     </AuthContext.Provider>
   );