src/db/
  schema.rs           # Single source of truth for table/column definitions
  query_builder.rs    # Fluent API for SQL construction
  repository.rs       # Base repository pattern
  migrations.rs       # Version-controlled schema changes (uses QueryBuilder)
  *_repository.rs     # Entity-specific data access

// GOOD: Using QueryBuilder
let sql = QueryBuilder::select(&TOOLS_TABLE)
    .columns(&["id", "display_name"])
    .where_eq("enabled")
    .order_by("display_name", true)
    .build();

// GOOD: Schema-driven DDL
let ddl = TOOLS_TABLE.create_table_sql();

// BAD: Hardcoded SQL
db.execute("SELECT * FROM tools WHERE id = ?", [id]).await?;

// BAD: Hardcoded DDL
db.execute("CREATE TABLE tools (id TEXT PRIMARY KEY)", ()).await?;

// BAD: SQL in migrations
db.execute("INSERT INTO schema_migrations...", params).await?;