feat(04-01): add tasks migration and sqlc query source

- CREATE TYPE task_status ENUM (todo, in_progress, in_review, done)
- CREATE TABLE tasks with tablo_id FK, position, status columns
- DROP order: table before type in Down migration (Pitfall 3)
- sqlc queries: ListTasksByTablo, InsertTask, GetTaskByID, UpdateTask, DeleteTask, MaxPositionByTabloAndStatus
- migration applied cleanly, sqlc generate produces TaskStatus type and Task struct

backend/internal/db/queries/tasks.sql

@@ -0,0 +1,29 @@
+-- name: ListTasksByTablo :many
+SELECT id, tablo_id, title, description, status, position, created_at, updated_at
+FROM tasks
+WHERE tablo_id = $1
+ORDER BY status, position, created_at;
+
+-- name: InsertTask :one
+INSERT INTO tasks (tablo_id, title, description, status, position)
+VALUES ($1, $2, $3, $4, $5)
+RETURNING id, tablo_id, title, description, status, position, created_at, updated_at;
+
+-- name: GetTaskByID :one
+SELECT id, tablo_id, title, description, status, position, created_at, updated_at
+FROM tasks
+WHERE id = $1 AND tablo_id = $2;
+
+-- name: UpdateTask :one
+UPDATE tasks
+SET title = $2, description = $3, status = $4, position = $5, updated_at = now()
+WHERE id = $1
+RETURNING id, tablo_id, title, description, status, position, created_at, updated_at;
+
+-- name: DeleteTask :exec
+DELETE FROM tasks WHERE id = $1 AND tablo_id = $2;
+
+-- name: MaxPositionByTabloAndStatus :one
+SELECT COALESCE(MAX(position), 0)::integer AS max_position
+FROM tasks
+WHERE tablo_id = $1 AND status = $2;

backend/migrations/0004_tasks.sql

@@ -0,0 +1,25 @@
+-- migrations/0004_tasks.sql
+-- Phase 4: Tasks (Kanban)
+
+-- +goose Up
+
+-- ENUM declaration order must match visual left-to-right column order (Pitfall 6).
+CREATE TYPE task_status AS ENUM ('todo', 'in_progress', 'in_review', 'done');
+
+CREATE TABLE tasks (
+    id          uuid        PRIMARY KEY DEFAULT gen_random_uuid(),
+    tablo_id    uuid        NOT NULL REFERENCES tablos(id) ON DELETE CASCADE,
+    title       text        NOT NULL,
+    description text,
+    status      task_status NOT NULL DEFAULT 'todo',
+    position    integer     NOT NULL DEFAULT 100,
+    created_at  timestamptz NOT NULL DEFAULT now(),
+    updated_at  timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE INDEX tasks_tablo_id_status_idx ON tasks(tablo_id, status, position);
+
+-- +goose Down
+-- Table MUST be dropped before type (Pitfall 3 — type is still referenced by table column).
+DROP TABLE IF EXISTS tasks;
+DROP TYPE IF EXISTS task_status;