3. Write the service

// backend/src/tasks/tasks.service.ts
import { Inject, Injectable } from '@nestjs/common';
import type { Knex } from 'knex';

import { KNEX_CONNECTION } from 'src/knex/knex.module';
import { TaskRow } from 'src/db/types';
import { paginate } from 'src/common/pagination';

@Injectable()
export class TasksService {
  constructor(@Inject(KNEX_CONNECTION) private readonly db: Knex) {}

  async findByUserPaginated(
    userId: string,
    offset: number,
    limit: number,
  ): Promise<{ items: TaskRow[]; total: number }> {
    // The count query carries the same filter as the list, minus paging.
    return paginate<TaskRow>(
      this.db<TaskRow>('tasks')
        .where({ user_id: userId })
        .orderBy('created_at', 'desc')
        .offset(offset)
        .limit(limit),
      this.db('tasks').where({ user_id: userId }),
    );
  }

  async findById(id: string): Promise<TaskRow | undefined> {
    return this.db<TaskRow>('tasks').where({ id }).first();
  }

  async create(input: { title: string; userId: string }): Promise<TaskRow> {
    const [row] = await this.db<TaskRow>('tasks')
      .insert({ title: input.title, user_id: input.userId })
      .returning('*');
    return row;
  }

  async update(input: {
    id: string;
    title?: string;
    completed?: boolean;
  }): Promise<TaskRow | undefined> {
    const { id, ...set } = input;
    if (Object.keys(set).length === 0) return this.findById(id);
    const [row] = await this.db<TaskRow>('tasks')
      .where({ id })
      .update({
        title: set.title,
        completed: set.completed,
        updated_at: new Date(),
      })
      .returning('*');
    return row;
  }

  async delete(id: string): Promise<boolean> {
    const n = await this.db<TaskRow>('tasks').where({ id }).delete();
    return n > 0;
  }
}

Every list goes through paginate() from src/common/pagination, as ItemsService.findAllPaginated does: it runs both queries and coerces the count, which pg returns as a string.