tools/local-dev: docker-compose stack for UI development

Adds tools/local-dev/ with postgres + redis + mailpit + backend +
gateway plus a Make wrapper, so `make -C tools/local-dev up` brings
the full authenticated stack online and `pnpm -C ui/frontend dev`
talks to it directly. The committed `.env.development` already
points at the stack and pins the matching gateway response public
key from the dev keypair under tools/local-dev/keys/.

The backend ships a new opt-in env, BACKEND_AUTH_DEV_FIXED_CODE
(`tools/local-dev/.env` defaults it to 123456). When set,
ConfirmEmailCode accepts that literal in addition to the real
bcrypt-verified code; SendEmailCode still queues a real email so
Mailpit captures the issued code at http://localhost:8025/, and
both paths coexist. The override is rejected as non-six-digit by
config validation and emits a loud warning at backend startup.

The local-dev Dockerfiles mirror backend/Dockerfile and
gateway/Dockerfile but switch the runtime stage to alpine so
docker-compose healthchecks can wget /healthz; the gateway
Dockerfile additionally copies ui/core/ into the build context
because gateway/go.mod's `replace galaxy/core => ../ui/core` is
required to compile the gateway main.

Smoke tested:
- `make -C tools/local-dev up` boots all five services to healthy.
- send-email-code + confirm-email-code with code=123456 returns a
  device_session_id; a real code in Mailpit also redeems
  successfully.
- `pnpm test` 14/14, `pnpm exec playwright test` 44/44.
- `go test ./backend/internal/config/...` green.

Docs: tools/local-dev/README.md, tools/local-dev/keys/README.md,
new "Local development stack" section in ui/docs/testing.md, and a
short pointer in ui/README.md.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/internal/config/config.go

@@ -71,6 +71,7 @@ const (
 	envAuthChallengeThrottleWindow = "BACKEND_AUTH_CHALLENGE_THROTTLE_WINDOW"
 	envAuthChallengeThrottleMax = "BACKEND_AUTH_CHALLENGE_THROTTLE_MAX"
 	envAuthUserNameMaxRetries = "BACKEND_AUTH_USERNAME_MAX_RETRIES"
+	envAuthDevFixedCode = "BACKEND_AUTH_DEV_FIXED_CODE"
 
 	envLobbySweeperInterval = "BACKEND_LOBBY_SWEEPER_INTERVAL"
 	envLobbyPendingRegistrationTTL = "BACKEND_LOBBY_PENDING_REGISTRATION_TTL"
@@ -293,6 +294,16 @@ type AuthConfig struct {
 	ChallengeMaxAttempts int
 	ChallengeThrottle AuthChallengeThrottleConfig
 	UserNameMaxRetries int
+
+	// DevFixedCode, when non-empty, makes ConfirmEmailCode accept this
+	// literal as a valid code in addition to the bcrypt-verified one
+	// stored on the challenge row. The override is intended for the
+	// `tools/local-dev` stack so a developer can log in without
+	// reading codes out of Mailpit. The variable MUST stay unset in
+	// production: validation requires a six-digit decimal value, and
+	// the auth service emits a loud startup warning when it picks the
+	// override up.
+	DevFixedCode string
 }
 
 // AuthChallengeThrottleConfig bounds how many un-consumed, non-expired
@@ -566,6 +577,7 @@ func LoadFromEnv() (Config, error) {
 	if cfg.Auth.UserNameMaxRetries, err = loadInt(envAuthUserNameMaxRetries, cfg.Auth.UserNameMaxRetries); err != nil {
 		return Config{}, err
 	}
+	cfg.Auth.DevFixedCode = loadString(envAuthDevFixedCode, cfg.Auth.DevFixedCode)
 
 	if cfg.Lobby.SweeperInterval, err = loadDuration(envLobbySweeperInterval, cfg.Lobby.SweeperInterval); err != nil {
 		return Config{}, err
@@ -745,6 +757,11 @@ func (c Config) Validate() error {
 	if c.Auth.UserNameMaxRetries <= 0 {
 		return fmt.Errorf("%s must be positive", envAuthUserNameMaxRetries)
 	}
+	if c.Auth.DevFixedCode != "" {
+		if !isDecimalString(c.Auth.DevFixedCode, 6) {
+			return fmt.Errorf("%s must be a six-digit decimal string when set", envAuthDevFixedCode)
+		}
+	}
 
 	if c.Lobby.SweeperInterval <= 0 {
 		return fmt.Errorf("%s must be positive", envLobbySweeperInterval)
@@ -809,6 +826,18 @@ func (c Config) Validate() error {
 	return nil
 }
 
+func isDecimalString(value string, length int) bool {
+	if len(value) != length {
+		return false
+	}
+	for _, r := range value {
+		if r < '0' || r > '9' {
+			return false
+		}
+	}
+	return true
+}
+
 func loadString(name, fallback string) string {
 	raw, ok := os.LookupEnv(name)
 	if !ok {