align types between sqlite & postgres

this one's achieved by adding "NOT NULL" at the *correct* position in
SQLite's create tables.

migrations/sqlite/2018-09-02-111458_create_medias/up.sql

@@ -1,6 +1,6 @@
 -- Your SQL goes here
 CREATE TABLE medias (
-    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
     file_path TEXT NOT NULL DEFAULT '',
     alt_text TEXT NOT NULL DEFAULT '',
     is_remote BOOLEAN NOT NULL DEFAULT 'f',