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.

tests/test_civitai_fetch.py

@@ -1,5 +1,6 @@
 from pathlib import Path
 
+from gpu_rent.civitai import pick_preview_image
 from gpu_rent.remote.civitai_fetch import fmt_bytes, progress_line, should_skip
 
 
@@ -54,3 +55,28 @@ def test_progress_line_unknown_total():
     line = progress_line("file", 1024 * 1024, None, 100_000)
     assert "%" not in line
     assert "/s" in line
+
+
+def test_pick_preview_image_first_jpeg():
+    version = {
+        "images": [
+            {"type": "video", "url": "https://example.com/x.mp4"},
+            {"type": "image", "url": "https://image.civitai.com/x/width=450/abc.jpeg"},
+        ]
+    }
+    got = pick_preview_image(version)
+    assert got is not None
+    url, suffix = got
+    assert "abc.jpeg" in url
+    assert suffix == ".preview.jpg"
+
+
+def test_pick_preview_image_png_and_default():
+    assert pick_preview_image({"images": [{"url": "https://cdn.example/a.png"}]})[1] == (
+        ".preview.png"
+    )
+    assert pick_preview_image({"images": [{"url": "https://cdn.example/hash/width=450"}]})[
+        1
+    ] == ".preview.jpg"
+    assert pick_preview_image({"images": []}) is None
+    assert pick_preview_image({}) is None