Refactor db code. Add migration support

internal/repository/migrations/0000_schema.down.sql

@@ -0,0 +1,5 @@
+CREATE TABLE authors (
+    id INTEGER PRIMARY KEY,
+    name TEXT NOT NULL,
+    bio TEXT
+);

internal/repository/migrations/0000_schema.up.sql

@@ -0,0 +1,11 @@
+CREATE TABLE news (
+    id INTEGER PRIMARY KEY,
+    name TEXT NOT NULL,
+    bio TEXT
+);
+
+CREATE TABLE retailer (
+    id INTEGER PRIMARY KEY,
+    name TEXT NOT NULL,
+    bio TEXT
+);