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/doctor.py

@@ -349,17 +349,31 @@ def _local_manifests(cfg: Config, checks: list[Check]) -> None:
 
     try:
         repos = parse_extensions(cfg.extensions_manifest)
-        if cfg.extensions_manifest.is_file():
-            checks.append(
-                Check(
-                    "extensions.yaml",
-                    True,
-                    True,
-                    f"{len(repos)} git-реп",
-                )
-            )
-        else:
+        if not cfg.extensions_manifest.is_file():
             checks.append(Check("extensions.yaml", True, False, "файла нет — стоковый SwarmUI"))
+        elif repos:
+            checks.append(Check("extensions.yaml", True, True, f"{len(repos)} git-реп"))
+        else:
+            example = cfg.extensions_manifest.with_name("extensions.example.yaml")
+            n_ex = 0
+            if example.is_file():
+                try:
+                    n_ex = len(parse_extensions(example))
+                except ConfigError:
+                    n_ex = 0
+            if n_ex:
+                checks.append(
+                    Check(
+                        "extensions.yaml",
+                        True,
+                        False,
+                        f"пустой; в example {n_ex} реп — скопируй и gpu-rent seed-extensions",
+                    )
+                )
+            else:
+                checks.append(
+                    Check("extensions.yaml", True, False, "0 реп — стоковый SwarmUI")
+                )
     except ConfigError as exc:
         checks.append(Check("extensions.yaml", False, True, str(exc)))