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/routes/handlers/misc_handlers.py

@@ -55,7 +55,7 @@ from ...utils.constants import (
     VALID_LORA_TYPES,
     VALID_OTHER_CIVITAI_TYPES,
 )
-from .hf_handlers import HfHandler
+from .model_source_handlers import ModelSourceHandler
 from .agent_handlers import AgentHandler
 from .download_routing_handlers import DownloadRoutingHandler
 from .model_handlers import ModelCivitaiHandler
@@ -4001,7 +4001,7 @@ class MiscHandlerSet:
         doctor: DoctorHandler,
         example_workflows: ExampleWorkflowsHandler,
         base_model: BaseModelHandlerSet,
-        hf_handler: Any = None,
+        model_source_handler: Any = None,
         agent_handler: Any = None,
         download_routing: Any = None,
     ) -> None:
@@ -4022,7 +4022,7 @@ class MiscHandlerSet:
         self.doctor = doctor
         self.example_workflows = example_workflows
         self.base_model = base_model
-        self.hf_handler = hf_handler
+        self.model_source_handler = model_source_handler
         self.agent_handler = agent_handler
         self.download_routing = download_routing
 
@@ -4076,10 +4076,13 @@ class MiscHandlerSet:
             "get_example_workflows": self.example_workflows.get_example_workflows,
             "get_example_workflow": self.example_workflows.get_example_workflow,
             # Hugging Face handlers
-            "get_hf_repo_files": self.hf_handler.get_hf_repo_files,
-            "download_hf_model": self.hf_handler.download_hf_model,
-            "set_hf_url": self.hf_handler.set_hf_url,
-            "get_model_sources": self.hf_handler.get_model_sources,
+            # External model sources (Hugging Face / ModelScope)
+            "list_model_source_files": self.model_source_handler.list_model_source_files,
+            "download_model_source": self.model_source_handler.download_model_source,
+            "get_hf_repo_files": self.model_source_handler.list_model_source_files,
+            "download_hf_model": self.model_source_handler.download_model_source,
+            "set_hf_url": self.model_source_handler.set_hf_url,
+            "get_model_sources": self.model_source_handler.get_model_sources,
             # Agent skill handlers
             "get_agent_skills": self.agent_handler.get_agent_skills,
             "execute_agent_skill": self.agent_handler.execute_agent_skill,