Add preview image handling for Civitai models and enhance job processing

- Introduced the `pick_preview_image` function to extract the first usable preview image URL and its suffix from model version data.
- Updated the `seed_civitai` function to include preview image URLs and destinations in job definitions, improving model handling.
- Implemented the `ensure_preview` function to download missing preview images during job processing, enhancing user experience.
- Added tests for `pick_preview_image` to ensure correct functionality across various scenarios, ensuring robustness in image handling.

src/gpu_rent/civitai.py

@@ -80,6 +80,39 @@ def pick_primary_file(version: dict) -> dict | None:
     return None
 
 
+def pick_preview_image(version: dict) -> tuple[str, str] | None:
+    """First usable Civitai preview → (url, sidecar_suffix) for SwarmUI.
+
+    SwarmUI looks for ``stem.preview.jpg`` / ``.preview.png`` / ``.jpg`` etc.
+    next to the weight (T2IModelHandler AutoImageFormatSuffixes).
+    """
+    images = version.get("images") or []
+    if not isinstance(images, list):
+        return None
+    for item in images:
+        if not isinstance(item, dict):
+            continue
+        kind = str(item.get("type") or "image").lower()
+        if kind and kind not in {"image", "img", ""}:
+            continue
+        url = str(item.get("url") or "").strip()
+        if not url.startswith("http"):
+            continue
+        path = url.split("?", 1)[0].lower()
+        if path.endswith(".png"):
+            suffix = ".preview.png"
+        elif path.endswith(".jpeg") or path.endswith(".jpg"):
+            suffix = ".preview.jpg"
+        elif path.endswith(".webp"):
+            # SwarmUI auto-format list has no .preview.webp — use .jpg name;
+            # most Civitai CDN URLs are jpeg without extension.
+            suffix = ".preview.jpg"
+        else:
+            suffix = ".preview.jpg"
+        return url, suffix
+    return None
+
+
 def fetch_model_version(token: str, host: str, version_id: int, timeout: float = 30.0) -> tuple[str, dict]:
     """GET /api/v1/model-versions/{id}; one retry on the other Civitai host."""
     first = _normalize_host(host)