#1: implement F0 foundation - monorepo, migrations, Go domain layer, service skeletons, CI

Stands up the LambdaMail monorepo per PLAN.md's F0 milestone: local dev
scaffolding (.env.example, Makefile, .gitignore excluding PLAN.md/docs/
.superpowers/), the full initial Postgres schema as a golang-migrate
migration, the complete Go domain layer for services/protocols (value
objects, entities, domain events - zero external imports beyond
github.com/google/uuid) with unit tests, health-checked service skeletons
for auth/storage (TypeScript) and webmail (Next.js), a local
docker-compose.yaml wiring all services together, a language-enforcement
lint script, and a GitHub Actions CI workflow.

Built via 11 sequential tasks under subagent-driven development, each with
an independent implementer + reviewer cycle. Fixes applied during review:
DomainName.IsPunycode() added to satisfy the promised interface; ioredis
default-import typing under NodeNext; next.js bumped 15.0.3->15.5.21 to
resolve high/critical CVEs; lang-lint.mjs corrected to not flag its own
banned-terms list; web service healthcheck added so compose --wait
actually waits on it; webmail Dockerfile's missing public/ directory:
protocols go.mod version aligned with CI's Go 1.22 pin.

Known follow-ups for later phases (not blocking F0): go-arch-lint isn't
wired into CI yet; live docker compose up and migration apply have not
been exercised in this dev environment (no Docker/psql available here)
and will get their first real run in CI, which does have Docker.

services/protocols/internal/domain/entity/dkim_key_pair.go

@@ -0,0 +1,32 @@
+package entity
+
+import "github.com/google/uuid"
+
+// DkimStatus is the lifecycle state of a DkimKeyPair (PLAN.md section 9, dkim_keys.status).
+type DkimStatus int
+
+const (
+	DkimStatusPending DkimStatus = iota
+	DkimStatusActive
+	DkimStatusRetiring
+	DkimStatusRevoked
+)
+
+// DkimKeyPair is a DKIM signing key with a selector (PLAN.md section 3).
+// Invariant enforced at the aggregate boundary (Domain), not here: only one
+// ACTIVE key per (domain, algorithm) - see the unique partial index in migrations/0001.
+type DkimKeyPair struct {
+	domainID uuid.UUID
+	selector string
+	status   DkimStatus
+}
+
+func NewDkimKeyPair(domainID uuid.UUID, selector string) *DkimKeyPair {
+	return &DkimKeyPair{domainID: domainID, selector: selector, status: DkimStatusPending}
+}
+
+func (k *DkimKeyPair) Selector() string   { return k.selector }
+func (k *DkimKeyPair) Status() DkimStatus { return k.status }
+func (k *DkimKeyPair) Activate()          { k.status = DkimStatusActive }
+func (k *DkimKeyPair) Retire()            { k.status = DkimStatusRetiring }
+func (k *DkimKeyPair) Revoke()            { k.status = DkimStatusRevoked }