refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/domain/cap/runtime_settings.go

@@ -0,0 +1,236 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package cap
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"strconv"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"golang.org/x/sync/singleflight"
+
+	"github.com/Rain-kl/Wavelet/backend/pkg/util"
+	cachepkg "github.com/Rain-kl/Wavelet/backend/plugins/infra/cache"
+	database "github.com/Rain-kl/Wavelet/backend/plugins/infra/database"
+)
+
+const (
+	defaultChallengeCount      = 1
+	defaultChallengeSize       = 32
+	defaultChallengeDifficulty = 4
+	defaultChallengeTTL        = 10 * time.Minute
+	defaultTokenTTL            = 20 * time.Minute
+)
+
+// RuntimeSettings is the parsed CAPTCHA runtime configuration loaded from system_configs.
+type RuntimeSettings struct {
+	LoginEnabled        bool
+	ChallengeCount      int
+	ChallengeSize       int
+	ChallengeDifficulty int
+	ChallengeTTL        time.Duration
+	TokenTTL            time.Duration
+}
+
+// CAP 动态配置键常量
+const (
+	ConfigKeyCapLoginEnabled        = "cap_login_enabled"
+	ConfigKeyCapChallengeCount      = "cap_challenge_count"
+	ConfigKeyCapChallengeSize       = "cap_challenge_size"
+	ConfigKeyCapChallengeDifficulty = "cap_challenge_difficulty"
+	ConfigKeyCapChallengeTTL        = "cap_challenge_ttl"
+	// ConfigKeyCapTokenTTL 验证码 Token 过期时间键
+	// #nosec G101
+	ConfigKeyCapTokenTTL = "cap_token_ttl"
+)
+
+var runtimeConfigKeys = []string{
+	ConfigKeyCapLoginEnabled,
+	ConfigKeyCapChallengeCount,
+	ConfigKeyCapChallengeSize,
+	ConfigKeyCapChallengeDifficulty,
+	ConfigKeyCapChallengeTTL,
+	ConfigKeyCapTokenTTL,
+}
+
+var runtimeConfigKeySet = func() map[string]struct{} {
+	set := make(map[string]struct{}, len(runtimeConfigKeys))
+	for _, key := range runtimeConfigKeys {
+		set[key] = struct{}{}
+	}
+	return set
+}()
+
+type runtimeSettingsStore struct {
+	snapshot     atomic.Pointer[RuntimeSettings]
+	loadGroup    singleflight.Group
+	listenerOnce sync.Once
+}
+
+var settingsStore = &runtimeSettingsStore{}
+
+// IsRuntimeConfigKey reports whether a system config key affects CAPTCHA runtime settings.
+func IsRuntimeConfigKey(key string) bool {
+	_, ok := runtimeConfigKeySet[key]
+	return ok
+}
+
+// CurrentSettings returns the cached CAPTCHA runtime settings snapshot.
+func CurrentSettings(ctx context.Context) (RuntimeSettings, error) {
+	return settingsStore.current(ctx)
+}
+
+// ProtectionEnabled reports whether CAPTCHA verification is required for protected routes.
+func ProtectionEnabled(ctx context.Context) bool {
+	settings, err := CurrentSettings(ctx)
+	if err != nil {
+		return false
+	}
+	return settings.LoginEnabled
+}
+
+// InvalidateRuntimeSettings drops the in-process CAPTCHA settings snapshot.
+func InvalidateRuntimeSettings() {
+	settingsStore.snapshot.Store(nil)
+}
+
+// ResetRuntimeSettingsForTest clears the CAPTCHA runtime snapshot.
+func ResetRuntimeSettingsForTest() {
+	InvalidateRuntimeSettings()
+}
+
+// InstallTestRuntimeSettings installs a fixed snapshot for unit tests.
+func InstallTestRuntimeSettings(settings RuntimeSettings) func() {
+	snapshot := settings
+	settingsStore.snapshot.Store(&snapshot)
+	return InvalidateRuntimeSettings
+}
+
+func (s *runtimeSettingsStore) current(ctx context.Context) (RuntimeSettings, error) {
+	s.ensureInvalidationListener()
+
+	if snapshot := s.snapshot.Load(); snapshot != nil {
+		return *snapshot, nil
+	}
+
+	loaded, err, _ := s.loadGroup.Do("cap-runtime-settings", func() (any, error) {
+		if snapshot := s.snapshot.Load(); snapshot != nil {
+			return *snapshot, nil
+		}
+
+		settings, loadErr := loadRuntimeSettings(ctx)
+		if loadErr != nil {
+			return RuntimeSettings{}, loadErr
+		}
+
+		s.snapshot.Store(&settings)
+		return settings, nil
+	})
+	if err != nil {
+		return RuntimeSettings{}, err
+	}
+
+	settings, ok := loaded.(RuntimeSettings)
+	if !ok {
+		return RuntimeSettings{}, errors.New("cap runtime settings loader returned unexpected type")
+	}
+	return settings, nil
+}
+
+func loadRuntimeSettings(ctx context.Context) (RuntimeSettings, error) {
+	type configRecord struct {
+		Key   string `gorm:"column:key"`
+		Value string `gorm:"column:value"`
+	}
+	var records []configRecord
+	if err := database.DB(ctx).Table("w_system_configs").Where("key IN ?", runtimeConfigKeys).Find(&records).Error; err != nil {
+		return RuntimeSettings{}, err
+	}
+	configs := make(map[string]string, len(records))
+	for _, r := range records {
+		configs[r.Key] = r.Value
+	}
+	return parseRuntimeSettings(configs), nil
+}
+
+func parseRuntimeSettings(configs map[string]string) RuntimeSettings {
+	settings := RuntimeSettings{
+		ChallengeCount:      defaultChallengeCount,
+		ChallengeSize:       defaultChallengeSize,
+		ChallengeDifficulty: defaultChallengeDifficulty,
+		ChallengeTTL:        defaultChallengeTTL,
+		TokenTTL:            defaultTokenTTL,
+	}
+
+	if val, ok := configs[ConfigKeyCapLoginEnabled]; ok {
+		if enabled, err := strconv.ParseBool(val); err == nil {
+			settings.LoginEnabled = enabled
+		}
+	}
+	if val, ok := configs[ConfigKeyCapChallengeCount]; ok {
+		if count, err := strconv.Atoi(val); err == nil && count > 0 {
+			settings.ChallengeCount = count
+		}
+	}
+	if val, ok := configs[ConfigKeyCapChallengeSize]; ok {
+		if size, err := strconv.Atoi(val); err == nil && size > 0 {
+			settings.ChallengeSize = size
+		}
+	}
+	if val, ok := configs[ConfigKeyCapChallengeDifficulty]; ok {
+		if diff, err := strconv.Atoi(val); err == nil && diff > 0 {
+			settings.ChallengeDifficulty = diff
+		}
+	}
+	if val, ok := configs[ConfigKeyCapChallengeTTL]; ok {
+		if ttlSeconds, err := strconv.Atoi(val); err == nil && ttlSeconds > 0 {
+			settings.ChallengeTTL = time.Duration(ttlSeconds) * time.Second
+		}
+	}
+	if val, ok := configs[ConfigKeyCapTokenTTL]; ok {
+		if ttlSeconds, err := strconv.Atoi(val); err == nil && ttlSeconds > 0 {
+			settings.TokenTTL = time.Duration(ttlSeconds) * time.Second
+		}
+	}
+
+	return settings
+}
+
+func (s *runtimeSettingsStore) ensureInvalidationListener() {
+	s.listenerOnce.Do(startRuntimeSettingsInvalidationListener)
+}
+
+// SystemConfigInvalidationChannel 系统配置失效广播通道
+const SystemConfigInvalidationChannel = "system_config:invalidation"
+
+func startRuntimeSettingsInvalidationListener() {
+	rdb := cachepkg.Redis
+	if rdb == nil {
+		return
+	}
+
+	util.Go(func() {
+		pubsub := rdb.Subscribe(context.Background(), SystemConfigInvalidationChannel)
+		defer func() {
+			_ = pubsub.Close()
+		}()
+
+		for msg := range pubsub.Channel() {
+			var payload struct {
+				Key string `json:"key"`
+			}
+			if err := json.Unmarshal([]byte(msg.Payload), &payload); err != nil {
+				InvalidateRuntimeSettings()
+				continue
+			}
+			if payload.Key == "" || payload.Key == "*" || IsRuntimeConfigKey(payload.Key) {
+				InvalidateRuntimeSettings()
+			}
+		}
+	})
+}