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/login/page.tsx

@@ -3,6 +3,7 @@
 import { useState } from "react";
 import { signInWithEmailAndPassword, GoogleAuthProvider, signInWithPopup } from "firebase/auth";
 import { auth } from "@/lib/firebase";
+import { c2api } from "@/lib/c2api";
 import { useRouter } from "next/navigation";
 
 export default function LoginPage() {
@@ -18,6 +19,7 @@ export default function LoginPage() {
     setError(null);
     try {
       await signInWithEmailAndPassword(auth, email, password);
+      c2api.recordSession().catch(() => {});
       router.push("/dashboard");
     } catch {
       setError("Invalid email or password.");
@@ -31,6 +33,7 @@ export default function LoginPage() {
     setError(null);
     try {
       await signInWithPopup(auth, new GoogleAuthProvider());
+      c2api.recordSession().catch(() => {});
       router.push("/dashboard");
     } catch {
       setError("Google sign-in failed. Try again.");