Add admin trend stats and sign-up sparkline

Task #13: Show admin trends like new sign-ups this week.

Backend
- Added GET /api/stats/admin (admin-only) in artifacts/api-server/src/routes/stats.ts.
- Returns: newUsersLast7Days, newUsersPrev7Days, activeServices,
  inactiveServices, signupsByDay (7-day series with zero-filled days,
  computed via date_trunc on usersTable.createdAt).

API spec / codegen
- Added /stats/admin path and AdminStats schema in lib/api-spec/openapi.yaml.
- Re-ran @workspace/api-spec codegen, regenerating react-query hooks
  (useGetAdminStats) and zod schemas.

Frontend (artifacts/teaboy-os/src/pages/admin.tsx)
- Wired useGetAdminStats in AdminPage (enabled only for admins).
- Extended DashboardSection with two trend cards (new users 7d w/
  delta vs previous 7d, active services with inactive count) and a
  small bar chart of sign-ups over the last 7 days.
- Added matching i18n keys (en/ar) under admin.dashboard.

Notes
- Avoided sending all users to the client for trend computation, as
  recommended in the task brief.
- Verified pnpm typecheck passes and the new endpoint returns 401 to
  unauthenticated callers.

Replit-Task-Id: dd11d7f7-5569-47b4-878e-ad63043eda31

lib/api-spec/openapi.yaml

@@ -704,6 +704,19 @@ paths:
               schema:
                 $ref: "#/components/schemas/HomeStats"
 
+  /stats/admin:
+    get:
+      operationId: getAdminStats
+      tags: [stats]
+      summary: Get admin dashboard trend stats
+      responses:
+        "200":
+          description: Admin stats
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/AdminStats"
+
 components:
   schemas:
     HealthStatus:
@@ -1287,3 +1300,33 @@ components:
         - unreadNotifications
         - unreadMessages
         - totalUsers
+
+    AdminStats:
+      type: object
+      properties:
+        newUsersLast7Days:
+          type: integer
+        newUsersPrev7Days:
+          type: integer
+        activeServices:
+          type: integer
+        inactiveServices:
+          type: integer
+        signupsByDay:
+          type: array
+          items:
+            type: object
+            properties:
+              date:
+                type: string
+              count:
+                type: integer
+            required:
+              - date
+              - count
+      required:
+        - newUsersLast7Days
+        - newUsersPrev7Days
+        - activeServices
+        - inactiveServices
+        - signupsByDay