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-c2-core/app/main.py

@@ -10,7 +10,7 @@ from app.internal.vocabulary_learner import vocabulary_induction_loop
 from app.internal.recorrelation_sweep import recorrelation_loop
 from app.config import settings
 from app.internal.auth import require_firebase_token, require_service_or_firebase_token
-from app.routers import nodes, systems, calls, upload, tokens, incidents, alerts, admin, trips, places, links
+from app.routers import nodes, systems, calls, upload, tokens, incidents, alerts, admin, trips, places, links, users
 from app.internal import firestore as fstore
 
 
@@ -72,6 +72,7 @@ app.include_router(trips.router,     dependencies=[Depends(require_service_or_fi
 app.include_router(places.router,    dependencies=[Depends(require_service_or_firebase_token)])
 app.include_router(upload.router)    # auth is per-node, handled inline
 app.include_router(admin.router)     # auth is per-endpoint (read: firebase, write: admin)
+app.include_router(users.router)     # auth: admin only
 app.include_router(links.router)     # auth is per-endpoint (generate: firebase, resolve: service key)