Refactor store: use migrations and split logic

internal/store/migrations/1.sql

@@ -0,0 +1,26 @@
+create table cake (
+    id integer primary key autoincrement,
+    name text not null unique,
+    price integer not null
+);
+
+create table customer_order (
+    id integer primary key autoincrement,
+    name text,
+    surname text,
+    phone text,
+    location text,
+    order_date string not null,
+    delivery_date string not null,
+    status text,
+    paid integer
+);
+
+create table ordered_cake (
+    customer_order integer references customer_order (id) not null,
+    cake integer references cake (id) not null,
+    amount integer not null,
+    primary key (customer_order, cake)
+);
+
+PRAGMA user_version = 1;