Add boards (aka. upper categories)

src/sql/schema.sql

@@ -4,9 +4,16 @@ create table if not exists users(
     hash varchar(128) not null
 );
 
+create table boards(
+    id serial primary key,
+    name varchar(32) not null unique,
+    description varchar(256) not null
+);
+
 create table if not exists categories(
     id serial primary key,
-    name varchar(32) not null unique
+    name varchar(32) not null unique,
+    board integer references boards(id)
 );
 
 create table if not exists tasks(