feat(03-01): create NewTestDB helper for in-memory SQLite test databases

- Add testhelper.go with exported NewTestDB(t *testing.T) *DB
- Opens :memory: SQLite with same connection params as production
- Shares applyPRAGMAs, schema application loop, and runMigrations
- Registers t.Cleanup for automatic DB close
- No orphan cleanup, no functional options, no error return

backend/database/testhelper.go

@@ -0,0 +1,74 @@
+package database
+
+import (
+	"database/sql"
+	"io/fs"
+	"log/slog"
+	"path"
+	"testing"
+
+	_ "modernc.org/sqlite" // Register sqlite driver.
+
+	"yellowjacket/backend/database/sql/sqlcgen"
+)
+
+// NewTestDB returns an in-memory SQLite database that mirrors the
+// production setup (PRAGMAs + all migrations). The database is
+// automatically closed when the test completes via t.Cleanup.
+func NewTestDB(t *testing.T) *DB {
+	t.Helper()
+
+	db, err := sql.Open(
+		"sqlite",
+		":memory:?_busy_timeout=5000&_journal_mode=WAL",
+	)
+	if err != nil {
+		t.Fatalf("could not open test database: %v", err)
+	}
+
+	db.SetMaxOpenConns(1)
+
+	ctx := t.Context()
+
+	if err := applyPRAGMAs(ctx, db); err != nil {
+		t.Fatalf("could not apply PRAGMAs: %v", err)
+	}
+
+	dirEntries, err := schemas.ReadDir("sql/schemas")
+	if err != nil {
+		t.Fatalf("could not read schemas directory: %v", err)
+	}
+
+	for _, dirEntry := range dirEntries {
+		if !dirEntry.IsDir() {
+			filePath := path.Join("sql/schemas", dirEntry.Name())
+
+			sqlContent, err := fs.ReadFile(schemas, filePath)
+			if err != nil {
+				t.Fatalf("could not read file %s: %v", filePath, err)
+			}
+
+			if _, err = db.ExecContext(ctx, string(sqlContent)); err != nil {
+				t.Fatalf(
+					"error executing sql from file %s: %v",
+					filePath, err,
+				)
+			}
+		}
+	}
+
+	if err := runMigrations(ctx, db, slog.Default()); err != nil {
+		t.Fatalf("could not run migrations: %v", err)
+	}
+
+	queries := sqlcgen.New(db)
+
+	t.Cleanup(func() { db.Close() })
+
+	return &DB{
+		db:      db,
+		Ctx:     ctx,
+		Queries: queries,
+		logger:  slog.Default(),
+	}
+}