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/__init__.py

@@ -12,10 +12,14 @@ from .base import (
     GROUP_PREFIXES,
     HTTP_TIMEOUT,
     ModelSource,
+    ModelSourceError,
     SourceRef,
     USER_AGENT,
     clean_source_url,
+    fetch_json,
     fetch_text,
+    filter_weight_files,
+    is_valid_source_id,
 )
 from .huggingface import HuggingFaceSource
 from .modelscope import ModelScopeSource
@@ -24,6 +28,8 @@ from .registry import (
     SOURCE_PLATFORM_FIELD,
     SOURCE_URL_FIELD,
     detect_source,
+    downloadable_sources,
+    get_download_source,
     get_source,
     get_source_platform,
     has_external_source,
@@ -40,6 +46,7 @@ __all__ = [
     "HTTP_TIMEOUT",
     "LEGACY_HF_URL_FIELD",
     "ModelSource",
+    "ModelSourceError",
     "HuggingFaceSource",
     "ModelScopeSource",
     "SOURCE_PLATFORM_FIELD",
@@ -49,10 +56,15 @@ __all__ = [
     "USER_AGENT",
     "clean_source_url",
     "detect_source",
+    "downloadable_sources",
+    "fetch_json",
     "fetch_text",
+    "filter_weight_files",
+    "get_download_source",
     "get_source",
     "get_source_platform",
     "has_external_source",
+    "is_valid_source_id",
     "list_sources",
     "normalize_metadata_source",
     "resolve_source_ref",