refactor: use Go 1.27 standard library uuid instead of satori/go.uuid

[whywaita]

Summary

Migrate the internal UUID implementation from the third-party github.com/satori/go.uuid package to the standard library uuid package introduced in Go 1.27.

Changes

UUID migration (API mapping)

• uuid.FromString(...) → uuid.Parse(...)
• uuid.FromStringOrNil(...) → uuid.MustParse(...) (test-only, fixed UUID strings)
• uuid.Equal(a, b) → a == b (std uuid.UUID is a comparable [16]byte)
• uuid.NewV4() / uuid.UUID / uuid.UUID{} / .String() are API-compatible
• go.mod: go 1.25 → go 1.27, drop the satori/go.uuid dependency

DB layer (mysql)
std uuid.UUID does not implement driver.Valuer / sql.Scanner (unlike satori). UUID columns are VARCHAR(36), so DB I/O is handled as strings:

• INSERT/WHERE: bind uuid.UUID.String() (CreateTarget, EnqueueJob)
• SELECT: scan into shadow structs whose UUID columns are strings, then parse back into datastore entities with uuid.Parse
• The domain type stays std uuid.UUID (no ripple into memory/web/runner/starter); DB conversion is confined to the mysql package

CI

• staticcheck@latest (v0.7.0) cannot decode Go 1.27 export data (version 4), so pin staticcheck to v0.8.0-rc.1
• Bump the Dockerfile builder image golang:1.25 → golang:1.27

Verification

• go build ./... / go vet ./... OK
• Full test suite (mysql/web incl.) passes in CI
• docker-build-test / docker-build-sha pass
• staticcheck / lint pass

[whywaita]

Be Go.

Suggested change

	func (r rowJob) job() (datastore.Job, error) {
	func (r rowJob) job() (*datastore.Job, error) {

[whywaita]

Addressed — this approach was replaced entirely. Instead of the shadow rowJob struct, I introduced datastore.UUID, which wraps std uuid.UUID and implements driver.Valuer + sql.Scanner. The datastore entity structs now use it, so sqlx binds/scans UUID columns directly and the mysql layer goes back to plain queries (no rowJob/rowRunner/rowTarget + string conversion). This makes the code cleaner and removes the need for this helper function altogether.