docs: add package comments to data layer packages

internal/database/database.go

@@ -1,3 +1,4 @@
+// Package database manages database schema migrations and fixes.
 package database
 
 import (

internal/database/fixes/registry.go

@@ -1,3 +1,4 @@
+// Package fixes implements one-off database migration fixes.
 package fixes
 
 import (

internal/db/helpers.go

@@ -1,3 +1,4 @@
+// Package db provides database access via sqlc-generated queries and helper functions.
 package db
 
 import "database/sql"

internal/domain/anime.go

@@ -1,3 +1,4 @@
+// Package domain defines the core domain types and interfaces used across the application.
 package domain
 
 import (