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.go

@@ -0,0 +1,37 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"fmt"
+)
+
+// WithTx runs fn in a transaction on the writer. It commits when fn returns
+// nil and rolls back when fn returns an error, panics or exits the goroutine.
+// A panic is not recovered: it propagates unchanged after the rollback.
+func (s *Store) WithTx(ctx context.Context, fn func(tx *sql.Tx) error) error {
+	tx, err := s.Writer.BeginTx(ctx, nil)
+	if err != nil {
+		return fmt.Errorf("store: begin transaction: %w", err)
+	}
+	finished := false
+	defer func() {
+		if !finished {
+			_ = tx.Rollback() // fn panicked; the panic carries on regardless
+		}
+	}()
+
+	if err := fn(tx); err != nil {
+		finished = true
+		if rbErr := tx.Rollback(); rbErr != nil {
+			return errors.Join(err, fmt.Errorf("store: roll back transaction: %w", rbErr))
+		}
+		return err
+	}
+	finished = true
+	if err := tx.Commit(); err != nil {
+		return fmt.Errorf("store: commit transaction: %w", err)
+	}
+	return nil
+}