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/modelscope.py

@@ -2,20 +2,38 @@
 
 ModelScope exposes the same "model card as README.md" convention as
 Hugging Face, including a YAML frontmatter block that often carries
-``base_model:`` and ``trigger_words:``.  Two public endpoints are used,
-neither of which requires an API key for public models:
+``base_model:`` and ``trigger_words:``.  Three public endpoints are used,
+none of which requires an API key for public models:
 
 * ``/models/{owner}/{name}/resolve/{revision}/README.md`` — raw model card
 * ``/api/v1/models/{owner}/{name}/repo?Revision=..&FilePath=README.md`` —
   the same content through the API, used as a fallback when the resolve
   URL is unavailable.
+* ``/api/v1/models/{owner}/{name}/repo/files?Revision=..`` — the file
+  listing backing the download picker.  It reports real sizes for LFS
+  files (not the pointer size), so no extra HEAD request is needed.
+
+Downloads go through ``/models/{owner}/{name}/resolve/{revision}/{path}``,
+which redirects to a CDN URL carrying a time-limited ``auth_key``.
+Requesting the resolve URL fresh on every attempt (which the shared
+downloader does, including for resumable Range requests) keeps that key
+valid; the CDN URL must never be cached.
 """
 
 from __future__ import annotations
 
+import logging
 import re
 
-from .base import ModelSource, fetch_text
+from .base import (
+    ModelSource,
+    ModelSourceError,
+    fetch_json,
+    fetch_text,
+    filter_weight_files,
+)
+
+logger = logging.getLogger(__name__)
 
 _URL_PATTERN = re.compile(
     r"https?://(?:www\.)?modelscope\.(?:cn|com)/models/(?P<id>[^/?#\s]+/[^/?#\s]+)"
@@ -41,7 +59,9 @@ class ModelScopeSource(ModelSource):
     platform = "modelscope"
     label = "ModelScope"
     supports_enrichment = True
-    supports_download = False
+    supports_download = True
+    default_revision = "master"
+    default_subdir = "modelscope"
     url_pattern = _URL_PATTERN
     strict_url_pattern = _STRICT_URL_PATTERN
 
@@ -49,7 +69,10 @@ class ModelScopeSource(ModelSource):
         return f"https://modelscope.cn/models/{source_id}"
 
     def asset_base_url(self, source_id: str, revision: str = "") -> str:
-        return f"https://modelscope.cn/models/{source_id}/resolve/{revision or 'master'}"
+        return (
+            f"https://modelscope.cn/models/{source_id}/resolve/"
+            f"{self.resolve_revision(revision)}"
+        )
 
     async def fetch_model_card(self, source_id: str) -> str:
         """Fetch the model card, preferring the raw resolve URL."""
@@ -72,5 +95,50 @@ class ModelScopeSource(ModelSource):
                 return text
         return ""
 
+    async def list_files(
+        self, source_id: str, revision: str = ""
+    ) -> list[dict]:
+        """List weight files via the repo files API.
+
+        ``master`` is the only branch name the API accepts — even repos
+        imported from Hugging Face are addressed as ``master`` (``main``
+        returns 404) — so no fallback probing is done here.
+        """
+
+        revision = self.resolve_revision(revision)
+        status, payload = await fetch_json(
+            "https://modelscope.cn/api/v1/models/"
+            f"{source_id}/repo/files?Revision={revision}"
+        )
+
+        if status == 404:
+            raise ModelSourceError(f"Repository '{source_id}' not found", status=404)
+        if status != 200 or not isinstance(payload, dict):
+            raise ModelSourceError(
+                f"ModelScope API error while listing '{source_id}' (HTTP {status})"
+            )
+
+        entries = []
+        for entry in (payload.get("Data") or {}).get("Files") or []:
+            if not isinstance(entry, dict) or entry.get("Type") != "blob":
+                continue
+            entries.append((entry.get("Path", ""), entry.get("Size", 0) or 0))
+
+        return filter_weight_files(entries)
+
+    def file_download_url(
+        self, source_id: str, filename: str, revision: str = ""
+    ) -> str:
+        return (
+            f"https://modelscope.cn/models/{source_id}/resolve/"
+            f"{self.resolve_revision(revision)}/{filename}"
+        )
+
+    def page_url_for_file(self, source_id: str, filename: str) -> str:
+        return (
+            f"https://modelscope.cn/models/{source_id}/file/view/"
+            f"{self.default_revision}/{filename}"
+        )
+
 
 __all__ = ["ModelScopeSource"]