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.controller.ts

@@ -43,4 +43,9 @@ export class UsersController {
   deactivate(@Param('id') id: string, @CurrentUser() actor: AuthenticatedUser) {
     return this.usersService.deactivate(id, actor.id);
   }
+
+  @Post(':id/activate')
+  activate(@Param('id') id: string, @CurrentUser() actor: AuthenticatedUser) {
+    return this.usersService.activate(id, actor.id);
+  }
 }

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' }),
+    );
+  });
 });

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

@@ -116,4 +116,24 @@ export class UsersService {
     });
     return this.toSummary(user);
   }
+
+  async activate(id: string, actingUserId?: string): Promise<UserSummary> {
+    const before = await this.findById(id);
+    if (before.isActive) {
+      throw new ConflictException('User is already active');
+    }
+    const user = await this.prisma.user.update({
+      where: { id },
+      data: { isActive: true, deactivatedAt: null },
+    });
+    await this.auditLog.record({
+      entityType: 'User',
+      entityId: id,
+      action: 'UPDATE',
+      userId: actingUserId,
+      beforeData: this.toSummary(before),
+      afterData: this.toSummary(user),
+    });
+    return this.toSummary(user);
+  }
 }