refactor(api): TOML 配置 SSOT、统一错误契约、Auth/事务加固与可观测性 (#33)

配置 SSOT（TOML + .env）
统一错误契约
Auth 与事务边界
Redis / Celery 可靠性:业务 Redis（DB/0）与 Celery broker/backend（DB/1）显式拆分；连接池、sync client
可观测性（OpenTelemetry + LGTM）

api/app/agents/image_prompt/orchestrator.py

@@ -12,6 +12,7 @@ from app.agents.image_prompt.prompt_agent import PromptGenerationAgent
 from app.core.config import settings
 from app.core.logging import get_logger
 from app.features.memoir.memoir_images.settings import MemoirImageSettings
+from app.features.memoir.constants import memoir
 
 logger = get_logger(__name__)
 
@@ -84,7 +85,7 @@ def get_image_prompt_orchestrator() -> ImagePromptOrchestrator:
     try:
         llm = LlmGateway().langchain_llm_for(LlmUseCase("image_prompt"))
     except Exception as e:
-        if settings.image_prompt_fallback_disabled:
+        if memoir.image_prompt_fallback_disabled:
             raise
         logger.warning(
             "ImagePromptOrchestrator LLM 初始化失败，使用确定性 fallback: {}",