feat(health): 被动式健康检查 API（FastAPI，覆盖服务健康+数据健康）

在同步进程内内置 FastAPI/uvicorn HTTP endpoint (GET /health)，返回结构化
健康快照，便于运维/监控被动探活：

- 服务健康：cycle_lag（核心指标，超过 3×poll_interval 判 unhealthy）、
  队列状态（pending/error/dead/cleaned 实时查）、capture 累计（enqueued/
  deferred/aged_out）、最近 cycle 时间/耗时/错误。
- 数据健康：解析最近一次定时 compare_ids 报告（不跑全量 compare 太重）、
  实时查 SyncLogArchive 异常痕迹（AgedOut/降级）、SyncQueue dead 行样本。

status 三态：healthy(200) / degraded(503, 有dead/error/aged_out/compare
不一致) / unhealthy(503, 主循环停滞)。状态码映射便于按码告警。

线程模型：uvicorn 占主线程，capture/apply/cleanup 循环跑后台 daemon 线程，
NSSM 停服务时主线程退出、daemon 自动终止。health.enabled=false 时退化为
旧行为（同步循环占主线程）。Web 层(routes/)与业务逻辑(HealthChecker)
解耦，后期加运维接口（compare触发/死信管理/metrics）零结构改动。

零 SQL/零 schema 改动。新增依赖 fastapi/uvicorn[standard]。

src/sync/web/app.py

@@ -0,0 +1,28 @@
+"""FastAPI application factory.
+
+``create_app(state, cfg)`` builds the app, binds the shared singletons (the
+process-wide :class:`ServiceState` and :class:`SyncConfig`) into the dependency
+graph, and mounts route modules. New operational endpoints are added by
+creating a ``routes/<name>.py`` with an ``APIRouter`` and ``include_router``-
+ing it here.
+"""
+from __future__ import annotations
+
+from fastapi import FastAPI
+
+from ..config import SyncConfig
+from ..health import ServiceState
+from . import deps
+from .routes import health as health_routes
+
+
+def create_app(state: ServiceState, cfg: SyncConfig) -> FastAPI:
+    """Build the FastAPI app with health state/config bound for injection."""
+    app = FastAPI(
+        title="DataMacroSync",
+        description="Access -> SQL Server incremental sync operational API",
+        version="1.0.0",
+    )
+    deps.bind(state, cfg)
+    app.include_router(health_routes.router)
+    return app