Add store helpers: transactions, ULIDs and time conventions

- Store.WithTx runs fn on the writer, committing on nil and rolling back
  on error, panic or goroutine exit; panics propagate unchanged.
- NewID returns a monotonic ULID string (oklog/ulid/v2, crypto/rand),
  strictly increasing within the process.
- Timestamp stores UTC RFC 3339 TEXT at fixed millisecond precision.
- LocalDate stores YYYY-MM-DD TEXT and rejects invalid or non-canonical
  dates.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/store/tx_test.go

@@ -0,0 +1,84 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"testing"
+)
+
+func openTxTestStore(t *testing.T) *Store {
+	t.Helper()
+	s := openTestStore(t)
+	if _, err := s.Writer.Exec("CREATE TABLE t (id INTEGER PRIMARY KEY)"); err != nil {
+		t.Fatal(err)
+	}
+	return s
+}
+
+func countRows(t *testing.T, s *Store) int {
+	t.Helper()
+	var n int
+	if err := s.Reader.QueryRow("SELECT count(*) FROM t").Scan(&n); err != nil {
+		t.Fatal(err)
+	}
+	return n
+}
+
+func insertRow(tx *sql.Tx) error {
+	_, err := tx.Exec("INSERT INTO t (id) VALUES (1)")
+	return err
+}
+
+func TestWithTxCommitsOnNil(t *testing.T) {
+	s := openTxTestStore(t)
+	if err := s.WithTx(context.Background(), insertRow); err != nil {
+		t.Fatal(err)
+	}
+	if n := countRows(t, s); n != 1 {
+		t.Fatalf("rows after commit = %d, want 1", n)
+	}
+}
+
+func TestWithTxRollsBackOnError(t *testing.T) {
+	s := openTxTestStore(t)
+	errBoom := errors.New("boom")
+	err := s.WithTx(context.Background(), func(tx *sql.Tx) error {
+		if err := insertRow(tx); err != nil {
+			t.Fatal(err)
+		}
+		return errBoom
+	})
+	if !errors.Is(err, errBoom) {
+		t.Fatalf("WithTx error = %v, want %v", err, errBoom)
+	}
+	if n := countRows(t, s); n != 0 {
+		t.Fatalf("rows after rollback = %d, want 0", n)
+	}
+}
+
+func TestWithTxRollsBackOnPanic(t *testing.T) {
+	s := openTxTestStore(t)
+	const value = "kaboom"
+	func() {
+		defer func() {
+			if p := recover(); p != value {
+				t.Fatalf("recovered %v, want panic %q to propagate", p, value)
+			}
+		}()
+		_ = s.WithTx(context.Background(), func(tx *sql.Tx) error {
+			if err := insertRow(tx); err != nil {
+				t.Fatal(err)
+			}
+			panic(value)
+		})
+		t.Fatal("WithTx returned instead of panicking")
+	}()
+	if n := countRows(t, s); n != 0 {
+		t.Fatalf("rows after panic = %d, want 0", n)
+	}
+	// The single writer connection must have been released.
+	if err := s.WithTx(context.Background(), insertRow); err != nil {
+		t.Fatalf("WithTx after panic: %v", err)
+	}
+}