feat(03-01): add tablos migration and sqlc queries

- 0003_tablos.sql: tablos table with user_id FK + ON DELETE CASCADE + tablos_user_id_idx
- tablos.sql: 5 named queries (ListTablosByUser, GetTabloByID, InsertTablo, UpdateTablo, DeleteTablo)
- UpdateTablo sets updated_at = now() explicitly (Pitfall 7)
- color not editable in UpdateTablo per Phase 3 scope
- sqlc generates Tablo struct with pgtype.Text for description/color (not committed per .gitignore convention)

backend/internal/db/queries/tablos.sql

@@ -0,0 +1,24 @@
+-- name: ListTablosByUser :many
+SELECT id, user_id, title, description, color, created_at, updated_at
+FROM tablos
+WHERE user_id = $1
+ORDER BY created_at DESC;
+
+-- name: GetTabloByID :one
+SELECT id, user_id, title, description, color, created_at, updated_at
+FROM tablos
+WHERE id = $1;
+
+-- name: InsertTablo :one
+INSERT INTO tablos (user_id, title, description, color)
+VALUES ($1, $2, $3, $4)
+RETURNING id, user_id, title, description, color, created_at, updated_at;
+
+-- name: UpdateTablo :one
+UPDATE tablos
+SET title = $2, description = $3, updated_at = now()
+WHERE id = $1
+RETURNING id, user_id, title, description, color, created_at, updated_at;
+
+-- name: DeleteTablo :exec
+DELETE FROM tablos WHERE id = $1;

backend/migrations/0003_tablos.sql

@@ -0,0 +1,18 @@
+-- migrations/0003_tablos.sql
+-- Phase 3: Tablos CRUD
+
+-- +goose Up
+CREATE TABLE tablos (
+    id          uuid        PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id     uuid        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    title       text        NOT NULL,
+    description text,
+    color       text,
+    created_at  timestamptz NOT NULL DEFAULT now(),
+    updated_at  timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE INDEX tablos_user_id_idx ON tablos(user_id);
+
+-- +goose Down
+DROP TABLE IF EXISTS tablos;