diplomail (Stage A): add in-game personal mail subsystem

Phase 28 of ui/PLAN.md needs a persistent player-to-player mail
channel; the existing `mail` package is a transactional email
outbox and the `notification` catalog is one-way platform events.
Stage A lands the schema (diplomail_messages / _recipients /
_translations), a single-recipient personal send/read/delete
service path, a `diplomail.message.received` push kind plumbed
through the notification pipeline, and an unread-counts endpoint
that drives the lobby badge. Admin / system mail, lifecycle hooks,
paid-tier broadcast, multi-game broadcast, bulk purge and language
detection / translation cache come in stages B–D.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/internal/config/config.go

@@ -96,6 +96,9 @@ const (
 	envNotificationWorkerInterval = "BACKEND_NOTIFICATION_WORKER_INTERVAL"
 	envNotificationMaxAttempts = "BACKEND_NOTIFICATION_MAX_ATTEMPTS"
 
+	envDiplomailMaxBodyBytes = "BACKEND_DIPLOMAIL_MAX_BODY_BYTES"
+	envDiplomailMaxSubjectBytes = "BACKEND_DIPLOMAIL_MAX_SUBJECT_BYTES"
+
 	envDevSandboxEmail = "BACKEND_DEV_SANDBOX_EMAIL"
 	envDevSandboxEngineImage = "BACKEND_DEV_SANDBOX_ENGINE_IMAGE"
 	envDevSandboxEngineVersion = "BACKEND_DEV_SANDBOX_ENGINE_VERSION"
@@ -163,6 +166,9 @@ const (
 	defaultNotificationWorkerInterval = 5 * time.Second
 	defaultNotificationMaxAttempts = 8
 
+	defaultDiplomailMaxBodyBytes = 4096
+	defaultDiplomailMaxSubjectBytes = 256
+
 	defaultDevSandboxEngineVersion = "0.1.0"
 	defaultDevSandboxPlayerCount = 20
 )
@@ -201,6 +207,7 @@ type Config struct {
 	Engine EngineConfig
 	Runtime RuntimeConfig
 	Notification NotificationConfig
+	Diplomail DiplomailConfig
 	DevSandbox DevSandboxConfig
 
 	// FreshnessWindow mirrors the gateway freshness window and is used by the
@@ -397,6 +404,22 @@ type RuntimeConfig struct {
 	StopGracePeriod time.Duration
 }
 
+// DiplomailConfig bounds the diplomatic-mail subsystem. Both limits
+// are enforced in the service layer, so they can be tuned at runtime
+// without a schema migration. Body and subject are stored as plain
+// UTF-8 text; HTML is neither parsed nor sanitised on the server.
+type DiplomailConfig struct {
+	// MaxBodyBytes caps the length of `diplomail_messages.body` in
+	// bytes (not runes). A send whose body exceeds the limit is
+	// rejected with ErrInvalidInput.
+	MaxBodyBytes int
+
+	// MaxSubjectBytes caps the length of `diplomail_messages.subject`
+	// in bytes. Subjects are optional; the empty-string default
+	// passes the limit trivially.
+	MaxSubjectBytes int
+}
+
 // NotificationConfig configures the notification fan-out module
 // implemented in `backend/internal/notification`. AdminEmail receives
 // admin-channel kinds (the `runtime.*` set in `backend/README.md` §10);
@@ -494,6 +517,10 @@ func DefaultConfig() Config {
 			WorkerInterval: defaultNotificationWorkerInterval,
 			MaxAttempts: defaultNotificationMaxAttempts,
 		},
+		Diplomail: DiplomailConfig{
+			MaxBodyBytes: defaultDiplomailMaxBodyBytes,
+			MaxSubjectBytes: defaultDiplomailMaxSubjectBytes,
+		},
 		DevSandbox: DevSandboxConfig{
 			EngineVersion: defaultDevSandboxEngineVersion,
 			PlayerCount: defaultDevSandboxPlayerCount,
@@ -657,6 +684,13 @@ func LoadFromEnv() (Config, error) {
 		return Config{}, err
 	}
 
+	if cfg.Diplomail.MaxBodyBytes, err = loadInt(envDiplomailMaxBodyBytes, cfg.Diplomail.MaxBodyBytes); err != nil {
+		return Config{}, err
+	}
+	if cfg.Diplomail.MaxSubjectBytes, err = loadInt(envDiplomailMaxSubjectBytes, cfg.Diplomail.MaxSubjectBytes); err != nil {
+		return Config{}, err
+	}
+
 	cfg.DevSandbox.Email = strings.TrimSpace(loadString(envDevSandboxEmail, cfg.DevSandbox.Email))
 	cfg.DevSandbox.EngineImage = strings.TrimSpace(loadString(envDevSandboxEngineImage, cfg.DevSandbox.EngineImage))
 	cfg.DevSandbox.EngineVersion = strings.TrimSpace(loadString(envDevSandboxEngineVersion, cfg.DevSandbox.EngineVersion))
@@ -853,6 +887,13 @@ func (c Config) Validate() error {
 	if c.Notification.MaxAttempts <= 0 {
 		return fmt.Errorf("%s must be positive", envNotificationMaxAttempts)
 	}
+
+	if c.Diplomail.MaxBodyBytes <= 0 {
+		return fmt.Errorf("%s must be positive", envDiplomailMaxBodyBytes)
+	}
+	if c.Diplomail.MaxSubjectBytes < 0 {
+		return fmt.Errorf("%s must not be negative", envDiplomailMaxSubjectBytes)
+	}
 	if email := strings.TrimSpace(c.Notification.AdminEmail); email != "" {
 		if _, err := netmail.ParseAddress(email); err != nil {
 			return fmt.Errorf("%s must be a valid RFC 5322 address: %w", envNotificationAdminEmail, err)