#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/mailbox.go

@@ -0,0 +1,36 @@
+package entity
+
+import (
+	"errors"
+
+	"github.com/google/uuid"
+	"lambdamail/protocols/internal/domain/valueobject"
+)
+
+// Mailbox is a mail account entity (PLAN.md section 3).
+type Mailbox struct {
+	domainID  uuid.UUID
+	address   valueobject.EmailAddress
+	hash      valueobject.PasswordHash
+	quota     valueobject.QuotaLimit
+	usedBytes int64
+}
+
+var ErrMailboxQuotaExceeded = errors.New("mailbox: recording this usage would exceed the quota")
+
+func NewMailbox(domainID uuid.UUID, address valueobject.EmailAddress, hash valueobject.PasswordHash, quota valueobject.QuotaLimit) *Mailbox {
+	return &Mailbox{domainID: domainID, address: address, hash: hash, quota: quota}
+}
+
+func (m *Mailbox) Address() valueobject.EmailAddress { return m.address }
+func (m *Mailbox) UsedBytes() int64                  { return m.usedBytes }
+func (m *Mailbox) Quota() valueobject.QuotaLimit      { return m.quota }
+
+// RecordUsage sets used_bytes, enforcing "used_bytes <= quota_bytes" (PLAN.md section 3).
+func (m *Mailbox) RecordUsage(bytes int64) error {
+	if m.quota.Exceeds(bytes) {
+		return ErrMailboxQuotaExceeded
+	}
+	m.usedBytes = bytes
+	return nil
+}