jimeng: add Volcengine Jimeng (即梦 AI) video provider with V4 signing

Implements the Jimeng/Volcengine part of issue #249, as agreed with
@xucailiang (who is handling the Kling provider separately).

Adds a first-class Jimeng video provider that calls the Volcengine
visual API directly (visual.volcengineapi.com) using HMAC-SHA256 V4
request signing with IAM AK/SK credentials. This is the first provider
in OpenMontage to use V4 signing (all others use Bearer token auth).

API flow: POST CVSync2AsyncSubmitTask -> poll CVSync2AsyncGetResult ->
download video_url.

Features:
- Text-to-video and image-to-video (Jimeng 3.0 Pro)
- Configurable frame count (121=5s, 241=10s at 24fps)
- Aspect ratio selection (16:9, 9:16, 1:1, etc.)
- Seed for reproducibility
- Full V4 HMAC-SHA256 request signing (not Bearer token)
- Error handling with Jimeng code 10000 success convention
- API key redaction in error messages (both env vars, no empty-string bug)

Env vars: VOLC_ACCESSKEY + VOLC_SECRETKEY (IAM AK/SK pair).
Idempotency keys include all output-affecting fields.

Files:
- tools/video/jimeng_video.py — new tool (V4 signing + submit/poll/download)
- tests/contracts/test_jimeng_video.py — 46 contract tests (no AK/SK needed)
- .env.example — VOLC_ACCESSKEY + VOLC_SECRETKEY
- docs/PROVIDERS.md — Volcengine Jimeng provider section

End-to-end tested with real Volcengine IAM credentials: generated a
1920x1088 H.264 5.04s video, ffprobe verified.

Test results:
  python -m pytest tests/contracts/test_jimeng_video.py -q  # 46 passed

docs/PROVIDERS.md

@@ -95,6 +95,44 @@ OpenMontage now uses those published rates in the Grok tool estimators.
 
 ---
 
+### Volcengine Jimeng — 即梦 AI Video Generation
+
+> **Direct ByteDance API via V4 signing.** Calls the Volcengine visual API (visual.volcengineapi.com) with HMAC-SHA256 request signing using IAM AK/SK credentials. Supports text-to-video and image-to-video via Jimeng 3.0 Pro.
+
+**Tools unlocked:** `jimeng_video`
+**Env vars:** `VOLC_ACCESSKEY` (Access Key ID) + `VOLC_SECRETKEY` (Secret Access Key)
+
+#### Setup
+
+1. Go to [console.volcengine.com/iam/keymanage](https://console.volcengine.com/iam/keymanage)
+2. Create a Volcengine account if you don't have one
+3. Create an Access Key pair (AK + SK)
+4. Ensure your account has access to Jimeng AI (即梦) video generation service
+5. Add to `.env`: `VOLC_ACCESSKEY=...` and `VOLC_SECRETKEY=...`
+
+#### What it's best for
+
+- Direct ByteDance/Volcengine API quota usage
+- Jimeng 3.0 Pro text-to-video and image-to-video
+- Chinese-language prompt understanding
+- Configurable frame count (121=5s, 241=10s) and aspect ratio
+
+#### API notes
+
+Authentication uses Volcengine IAM V4 signing (HMAC-SHA256), not a Bearer token. The signing process builds a canonical request, derives a signing key from SK → date → region → service, and signs the request.
+
+API flow: `POST ?Action=CVSync2AsyncSubmitTask` → poll `POST ?Action=CVSync2AsyncGetResult` → download `video_url`.
+
+The `req_key` for video is `jimeng_ti2v_v30_pro`. Success code is `10000`. Task statuses: `in_queue`, `generating`, `done`, `not_found`, `expired`.
+
+#### Pricing
+
+| Model | Price |
+|------|-------|
+| Jimeng 3.0 Pro (video) | ~$0.05/sec (check Volcengine console for actual rate) |
+
+---
+
 ### Alibaba DashScope — Qwen Image + TTS + ASR
 
 > **Best for Chinese-language production.** One key unlocks Qwen-Image generation, Qwen-TTS Mandarin narration, and Qwen-ASR with word-level timestamps — the only DashScope path that provides word-level granularity for subtitle alignment.