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/id_test.go

@@ -0,0 +1,25 @@
+package store
+
+import (
+	"testing"
+
+	"github.com/oklog/ulid/v2"
+)
+
+func TestNewIDStrictlyIncreasing(t *testing.T) {
+	const n = 10000
+	prev := NewID()
+	for range n {
+		id := NewID()
+		if len(id) != ulid.EncodedSize {
+			t.Fatalf("NewID() = %q, want %d characters", id, ulid.EncodedSize)
+		}
+		if _, err := ulid.ParseStrict(id); err != nil {
+			t.Fatalf("NewID() = %q: %v", id, err)
+		}
+		if id <= prev {
+			t.Fatalf("NewID() = %q, not after previous %q", id, prev)
+		}
+		prev = id
+	}
+}