Add ComfyUI image generation (/v1/images/generations)

OpenAI-compatible image endpoint that proxies to a ComfyUI server: fills a
workflow template with per-request params (prompt/size/n/seed/steps/checkpoint),
submits to ComfyUI, polls history, fetches images, returns OpenAI image shape.

- router.py: _handle_image + fill_workflow + route; env ROUTER_COMFYUI /
  ROUTER_COMFY_CHECKPOINT / ROUTER_COMFY_WORKFLOW
- comfyui-workflow.json: default SD/SDXL text2img template with {{SENTINELS}}
- docs/IMAGES.md + README: config, usage, custom workflows
- verified: templating + clean error when ComfyUI is down; end-to-end pending
  a live ComfyUI

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

docs/IMAGES.md

@@ -0,0 +1,62 @@
+# Image generation (ComfyUI)
+
+The router exposes an OpenAI-compatible **`POST /v1/images/generations`** endpoint
+backed by a [ComfyUI](https://github.com/comfyanonymous/ComfyUI) server. It submits
+a workflow to ComfyUI, polls until it finishes, fetches the images, and returns them
+in OpenAI's image-response shape — so any OpenAI image client works.
+
+## Configure
+
+| Env var | Default | Purpose |
+|---|---|---|
+| `ROUTER_COMFYUI` | `http://127.0.0.1:8188` | ComfyUI server URL |
+| `ROUTER_COMFY_CHECKPOINT` | *(unset)* | your checkpoint filename, e.g. `sd_xl_base_1.0.safetensors` |
+| `ROUTER_COMFY_WORKFLOW` | `comfyui-workflow.json` | the workflow template (see below) |
+
+At minimum, **set `ROUTER_COMFY_CHECKPOINT`** to a model that exists in your
+ComfyUI (`ComfyUI/models/checkpoints/`), or hardcode it into the template's
+`ckpt_name`. Then restart the router.
+
+## Use
+
+```bash
+curl http://<host>:8080/v1/images/generations -H 'content-type: application/json' -d '{
+  "prompt": "a red fox in a snowy forest, cinematic lighting",
+  "n": 1, "size": "1024x1024"
+}'
+# -> {"created": ..., "data": [{"b64_json": "<png>"}]}
+```
+- `response_format`: `b64_json` (default) or `url` (returns a `data:` URL).
+- Extra (non-OpenAI) knobs the router honors: `negative_prompt`, `steps`.
+- Works with the OpenAI SDK: `images.generate(prompt=..., size=..., n=...)`.
+
+## The workflow template
+
+`comfyui-workflow.json` is a ComfyUI **API-format** workflow with string sentinels
+the router substitutes per request:
+
+| Sentinel | Filled with |
+|---|---|
+| `{{PROMPT}}` | the request prompt |
+| `{{NEGATIVE}}` | `negative_prompt` |
+| `{{WIDTH}}` / `{{HEIGHT}}` | parsed from `size` |
+| `{{BATCH}}` | `n` |
+| `{{SEED}}` | a fresh seed |
+| `{{STEPS}}` | `steps` (default 25) |
+| `{{CHECKPOINT}}` | `ROUTER_COMFY_CHECKPOINT` |
+
+The default is a standard SD/SDXL text-to-image graph. **To use your own** (Flux,
+a fancier pipeline, LoRAs, upscalers, etc.): build it in ComfyUI, export via
+**Save (API Format)**, then replace the values you want driven by requests with the
+sentinels above. Point `ROUTER_COMFY_WORKFLOW` at your file. Anything without a
+sentinel stays fixed.
+
+## Notes
+
+- **Not yet verified end-to-end** — it was written before ComfyUI was installed.
+  Once your ComfyUI is up (`ROUTER_COMFYUI` reachable, checkpoint set), test with
+  the curl above; if a node name/shape differs, adjust the template.
+- Generation can take a while; the router polls up to ~10 min per request.
+- Video: ComfyUI video workflows (AnimateDiff, SVD, etc.) also export to API format
+  and return frames/files — the same endpoint can drive them, but the response
+  mapping for multi-frame/video output may need a tweak. Ask when you get there.