Add structured per-request access logging with a request id

Each request now emits one access line (bgfilter.access) carrying a
request_id, the effective params (source file/url, seg_model, cross_check,
background_mode, screen_color, auth), input/output sizes, and stage
timings (t_queue/fetch/decode/process/total_ms). A middleware binds the id
(reusing a sanitized inbound X-Request-ID, else uuid), a logging filter
injects it into every record, and it is echoed back as the X-Request-ID
response header on every response -- including errors, which also log the
detail. Requests that raise still produce a line (fields accumulate on
request.state; the middleware finalizes status/total/client).

request_id lives in a contextvar (not a thread-local) so a pooled worker
never inherits the previous request's id; propagating it into the executor
for deep pipeline logs is left for later. BGFILTER_LOG_FORMAT=text|json
switches human vs shipper-friendly output. /healthz is not logged.

Verified locally via TestClient (fake manager, no models): 200 path logs
all fields + timings with rid == header; inbound id echoed; over-long id
sanitized; XOR 400s log status + error.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

DEPLOY_ZH.md

@@ -200,6 +200,8 @@ export HF_HUB_DISABLE_XET=1                 # 这些 repo 是 Xet 存储；强
 | `BGFILTER_CONFIG` | `configs/default.yaml` | 管线配置文件 |
 | `BGFILTER_DEVICE` | `cpu` | 同时覆盖 model 和 segmentation 的 device（GPU 用 `cuda`）|
 | `BGFILTER_MAX_IMAGE_PIXELS` | `4194304` | 超过则返回 `413`（约 4 MP）|
+| `BGFILTER_LOG_FORMAT` | `text` | `text`（人读）或 `json`（每行一条结构化对象,便于日志采集）。每请求打一行访问日志,含 `request_id`、参数、分阶段耗时（`t_queue/fetch/decode/process/total_ms`）;该 id 同时以 `X-Request-ID` 响应头返回。 |
+| `BGFILTER_LOG_LEVEL` | `INFO` | 根日志级别。 |
 | `BGFILTER_FETCH_MAX_BYTES` | `26214400` | `image_url` 模式：下载体上限（约 25 MB）→ `413`。无 SSRF 过滤——务必开鉴权 / 调用方可信。 |
 | `BGFILTER_FETCH_TIMEOUT` | `15` | `image_url` 模式：fetch 读超时（秒；连接超时固定 5 秒）。出站 fetch 并发固定为 `2 × cpu_workers`（硬顶 64）。 |
 | `BGFILTER_PRELOAD` | `1` | 启动时预加载默认模型（首个请求不用冷加载）|