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/internal/auth.py

@@ -37,12 +37,28 @@ async def require_service_or_firebase_token(
         raise HTTPException(status_code=401, detail="Invalid or expired token")
 
 
+def get_role(decoded: dict) -> str:
+    """Extract the effective role from a decoded Firebase token.
+
+    Checks the granular ``role`` claim first, then falls back to the legacy
+    ``admin`` boolean so existing tokens continue to work during the transition.
+    """
+    if decoded.get("role") == "admin" or decoded.get("admin"):
+        return "admin"
+    role = decoded.get("role", "viewer")
+    return role if role in ("admin", "operator", "viewer") else "viewer"
+
+
 async def require_admin_token(
     credentials: Optional[HTTPAuthorizationCredentials] = Security(_bearer),
 ) -> dict:
-    """Verify a Firebase ID token AND require the admin custom claim."""
+    """Verify a Firebase ID token AND require the admin role.
+
+    Accepts both the legacy ``admin: True`` boolean claim and the newer
+    ``role: "admin"`` claim so tokens issued before the role migration still work.
+    """
     decoded = await require_firebase_token(credentials)
-    if not decoded.get("admin"):
+    if get_role(decoded) != "admin":
         raise HTTPException(status_code=403, detail="Admin access required")
     return decoded
 
@@ -77,7 +93,7 @@ async def require_service_key_or_admin(
         decoded = firebase_auth.verify_id_token(token)
     except Exception:
         raise HTTPException(status_code=401, detail="Invalid or expired token")
-    if not decoded.get("admin"):
+    if get_role(decoded) != "admin":
         raise HTTPException(status_code=403, detail="Admin access required")
     return decoded