Make db creation better

sql/card_stats.sql

@@ -0,0 +1,13 @@
+create table card_stats
+(
+    id          INTEGER
+        primary key autoincrement,
+    word_id     INTEGER           not null
+        references words
+            on delete cascade,
+    set_id      TEXT              not null
+        references card_set
+            on delete cascade,
+    score       INTEGER default 1 not null,
+    last_opened integer           not null
+);