feat(server): hot-reload config store and reloadable infra clients

- Replace single Load() with a watching Store that re-reads config files
  (and config.local.yaml) and fans out a ReloadEvent with a per-field diff
  so consumers can decide whether the change is hot-applicable or requires
  a process restart.
- Wrap llm, retrieval, vector store, and object storage clients in
  Reloadable* shells so the bootstrap can swap their underlying impls when
  config changes without re-instantiating handlers.
- Make jwt.Manager and ops TokenIssuer mutable under a lock so secrets and
  TTLs can be rotated live; thread default plan code through a setter on
  the ops AdminUserService.
- Wire ConfigStore through bootstrap and every cmd/main.go, scheduler /
  worker / tenant-api / ops-api start the watcher; services and handlers
  take a config.Provider so they always read current values for things
  like generation.stream_enabled, scheduler dispatch, retrieval, etc.
- Switch shared/config decoding off viper to a Kratos-derived runtime
  package so env placeholders (\${VAR:default}) resolve consistently and
  the same source machinery powers both the loader and the watcher.

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

server/internal/tenant/transport/asset_handler.go

@@ -20,15 +20,11 @@ import (
 )
 
 type AssetHandler struct {
-	secret string
-	app    *bootstrap.App
+	app *bootstrap.App
 }
 
 func NewAssetHandler(app *bootstrap.App) *AssetHandler {
-	return &AssetHandler{
-		secret: strings.TrimSpace(app.Config.JWT.Secret),
-		app:    app,
-	}
+	return &AssetHandler{app: app}
 }
 
 func (h *AssetHandler) BuildArticleImageURL(objectKey string) string {
@@ -109,12 +105,19 @@ func convertPublicAssetFormat(content []byte, format string) ([]byte, string, bo
 
 func (h *AssetHandler) signObjectKey(objectKey string) string {
 	encodedKey := base64.RawURLEncoding.EncodeToString([]byte(objectKey))
-	mac := hmac.New(sha256.New, []byte(h.secret))
+	mac := hmac.New(sha256.New, []byte(h.secret()))
 	_, _ = mac.Write([]byte(objectKey))
 	signature := base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
 	return fmt.Sprintf("%s.%s", encodedKey, signature)
 }
 
+func (h *AssetHandler) secret() string {
+	if h == nil || h.app == nil || h.app.Config() == nil {
+		return ""
+	}
+	return strings.TrimSpace(h.app.Config().JWT.Secret)
+}
+
 func (h *AssetHandler) parseToken(token string) (string, bool) {
 	parts := strings.Split(token, ".")
 	if len(parts) != 2 {