Add image_url input mode (service fetches the image)

/remove-background now accepts either an uploaded `file` or an `image_url`
the service fetches itself (exactly one required). The fetch runs BEFORE
the compute gate so a slow download never holds a pool slot, and is
bounded by its own semaphore (2 x cpu_workers, hard cap 64) plus a shared
httpx client (connection reuse, matching max_connections) -- this restores
the throttling a direct upload has implicitly, so a burst of tiny URL
requests can't exhaust FDs/ephemeral ports or hammer the upstream.

Bounds: 25 MB body cap (413), 5 s connect / 15 s read timeout, <=3
redirects; MAX_IMAGE_PIXELS still applies post-decode. Deliberately NO
SSRF filtering (no private-IP/host allowlist) -- the endpoint trusts
authenticated callers; auth runs before any fetch. Do not expose to
untrusted clients without adding private-address/redirect filtering to
_fetch_image.

Adds httpx. New env knobs BGFILTER_FETCH_MAX_BYTES / BGFILTER_FETCH_TIMEOUT.
Docs (README/README_ZH/DEPLOY/DEPLOY_ZH) updated. _fetch_image unit-tested
locally (happy path, size cap ->413, bad scheme ->400, 404 ->400);
on-box end-to-end still pending (test box went offline).

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

README_ZH.md

@@ -186,7 +186,8 @@ python -m uvicorn app:app --host 127.0.0.1 --port 18083 --workers 1
 
 | 字段 | 必填 | 取值 | 含义 |
 | --- | --- | --- | --- |
-| `file` | 是 | 图片文件 | 源图(字段名固定为 `file`)。 |
+| `file` | `file`/`image_url` 二选一 | 图片文件 | 以上传方式提供源图(字段名固定为 `file`)。 |
+| `image_url` | `file`/`image_url` 二选一 | `http(s)://…` | 以 URL 提供源图,服务端自行 fetch(受大小/超时限制,跟随 ≤3 跳重定向)。`file` 与 `image_url` 必须恰好提供一个。无 SSRF 过滤:端点信任已鉴权的调用方,请保持鉴权开启且调用方可信。 |
 | `background_mode` | 否 | `flat`(默认)/ `complex` | `flat` = 纯色背景(走色键管线)。`complex` = 非纯色/实景背景:无色键,完全由分割器驱动抠图;此模式下 `screen_color` 被忽略,发丝细节通常弱于纯色模式。 |
 | `screen_color` | 否 | `#RRGGBB`,或留空/`auto` | 背景色先验;留空则从边框自动探测。`background_mode=complex` 时忽略。 |
 | `seg_model` | 否 | `birefnet`(默认)/ `anime-seg` | 分割后端。 |