feat(download): support ModelScope repositories in the URL downloader

ModelScope became a linkable source, but downloading from it was impossible:
the URL picker only recognised huggingface.co, the file listing hit a
huggingface-only endpoint, the resolve URL was hardcoded, and the default
path template always wrote into a `huggingface/` directory.

Move the download knowledge into the providers so the handlers stay generic:

- `ModelSource` gains `list_files()`, `file_download_url()`,
  `default_revision` and `default_subdir`. `HuggingFaceSource` keeps the Hub
  tree API (`/api/models/{id}/tree/{rev}`, LFS-aware sizes, `main`).
  `ModelScopeSource` uses `/api/v1/models/{id}/repo/files?Revision=master`
  — which reports real byte sizes for LFS files, so no HEAD probe is needed,
  and which only accepts `master` (an HF-imported repo still 404s on `main`)
  — and downloads through `/models/{id}/resolve/{rev}/{path}`. That URL
  redirects to a CDN target carrying a time-limited `auth_key`, so it is
  rebuilt on every request and never cached, which is also what keeps
  resumable Range requests working.
- `hf_handlers.py`/`HfHandler` become `model_source_handlers.py`/
  `ModelSourceHandler` with `list_model_source_files` and
  `download_model_source`. New routes `/api/lm/model-source-files` and
  `/api/lm/download-model-source`; the old `/api/lm/hf-repo-files` and
  `/api/lm/download-hf-model` paths stay as aliases, and a payload without
  `platform` still means Hugging Face, so existing callers are unaffected.
- A downloaded sidecar now records `source_platform` + `source_url` (with the
  `hf_url` alias only for Hugging Face) instead of always writing `hf_url`,
  and `use_default_paths` files ModelScope downloads under
  `modelscope/<owner>/<repo>`. The now-unused shared HF aiohttp session and
  its shutdown hook are gone; providers open short-lived sessions.
- Frontend: `detectUrlType` returns the platform-neutral
  `model-source-repo` / `model-source-file` plus an explicit `platform`, the
  DownloadManager's `hf*` state and methods are renamed to `source*`, every
  `source === 'huggingface'` check becomes `isExternalModelSource()`, and
  batch groups are keyed by `platform:repo` so the same `owner/name` on two
  sites renders as two groups. A bare `owner/name` still means Hugging Face.
- `is_valid_source_id()` centralises repo-id validation (exactly
  `owner/name`, no traversal, no leading dot). This also fixes the old HF
  download check that rejected any dot in the name, i.e. legitimate repos
  such as `black-forest-labs/FLUX.1-dev`.

Verified against the live APIs: the example repo lists 8 weight files with
correct sizes, and a ranged GET of the built resolve URL returns 206 after
following the redirect to the CDN. Backend 2853 passed; frontend 1143 JS +
91 Vue passed. The nine locales carry the refreshed download copy in the
next commit.

py/services/model_sources/registry.py

@@ -56,6 +56,21 @@ def source_label(platform: Optional[str], default: str = "") -> str:
     return source.label if source else default
 
 
+def downloadable_sources() -> list[ModelSource]:
+    """Return the sources whose repositories can be downloaded directly."""
+
+    return [source for source in _SOURCES if source.supports_download]
+
+
+def get_download_source(platform: Optional[str]) -> Optional[ModelSource]:
+    """Return the source for *platform*, but only when it supports downloads."""
+
+    source = get_source(platform)
+    if source is None or not source.supports_download:
+        return None
+    return source
+
+
 def detect_source(url: Optional[str], *, strict: bool = False) -> Optional[SourceRef]:
     """Return the :class:`SourceRef` for *url*, or ``None`` if unsupported."""
 
@@ -197,6 +212,8 @@ __all__ = [
     "SOURCE_PLATFORM_FIELD",
     "SOURCE_URL_FIELD",
     "detect_source",
+    "downloadable_sources",
+    "get_download_source",
     "get_source",
     "get_source_platform",
     "has_external_source",