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>

go.mod

@@ -4,6 +4,7 @@ go 1.25.0
 
 require (
 	github.com/go-chi/chi/v5 v5.3.1
+	github.com/oklog/ulid/v2 v2.1.2
 	modernc.org/sqlite v1.59.0
 )