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.go

@@ -0,0 +1,27 @@
+package store
+
+import (
+	"crypto/rand"
+	"sync"
+	"time"
+
+	"github.com/oklog/ulid/v2"
+)
+
+var (
+	idMu      sync.Mutex
+	idEntropy = ulid.Monotonic(rand.Reader, 0)
+	idLastMS  uint64
+)
+
+// NewID returns a new ULID as its canonical 26-character string. IDs from
+// this process sort strictly in creation order, including within one
+// millisecond and across a backwards wall-clock step. It panics if the
+// monotonic entropy overflows within a millisecond.
+func NewID() string {
+	idMu.Lock()
+	defer idMu.Unlock()
+	ms := max(ulid.Timestamp(time.Now()), idLastMS)
+	idLastMS = ms
+	return ulid.MustNew(ms, idEntropy).String()
+}