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/app/nodes/page.tsx

@@ -1,15 +1,25 @@
 "use client";
 
-import { useState } from "react";
+import { useEffect, useState } from "react";
+import { useRouter } from "next/navigation";
 import { useNodes } from "@/lib/useNodes";
 import { useSystems } from "@/lib/useSystems";
 import { NodeCard } from "@/components/NodeCard";
 import { NodeConfigModal } from "@/components/NodeConfigModal";
+import { useAuth } from "@/components/AuthProvider";
 import type { NodeRecord } from "@/lib/types";
 
 export default function NodesPage() {
+  const { isAdmin, isOperator, loading: authLoading } = useAuth();
+  const router = useRouter();
   const { nodes, loading } = useNodes();
   const { systems } = useSystems();
+
+  useEffect(() => {
+    if (!authLoading && !isAdmin && !isOperator) router.replace("/dashboard");
+  }, [authLoading, isAdmin, isOperator, router]);
+
+  if (authLoading || (!isAdmin && !isOperator)) return null;
   const [configNode, setConfigNode] = useState<NodeRecord | null>(null);
 
   const systemMap = Object.fromEntries(systems.map((s) => [s.system_id, s]));