feat(backend): activate endpoint for deactivated users

Symmetrisch zu Modul-27/28-Fix fuer Colony/Apiary: kein Weg, einen
versehentlich deaktivierten Benutzer wieder zu aktivieren. Neuer
POST :id/activate-Endpunkt setzt isActive zurueck auf true und
loescht deactivatedAt; ConflictException wenn bereits aktiv.

ApiCore-Backend/src/users/users.service.spec.ts

@@ -122,4 +122,32 @@ describe('UsersService', () => {
       expect.objectContaining({ entityType: 'User', action: 'DEACTIVATE' }),
     );
   });
+
+  it('activate throws ConflictException when already active', async () => {
+    prisma.user.findUnique.mockResolvedValue(existingUser);
+    await expect(usersService.activate(existingUser.id)).rejects.toBeInstanceOf(
+      ConflictException,
+    );
+  });
+
+  it('activate sets isActive to true and clears deactivatedAt', async () => {
+    prisma.user.findUnique.mockResolvedValue({
+      ...existingUser,
+      isActive: false,
+      deactivatedAt: new Date(),
+    });
+    prisma.user.update.mockResolvedValue({
+      ...existingUser,
+      isActive: true,
+      deactivatedAt: null,
+    });
+
+    const result = await usersService.activate(existingUser.id);
+
+    expect(result.isActive).toBe(true);
+    expect(result.deactivatedAt).toBeNull();
+    expect(auditLog.record).toHaveBeenCalledWith(
+      expect.objectContaining({ entityType: 'User', action: 'UPDATE' }),
+    );
+  });
 });