feat(links): support ModelScope and TensorArt as model sources

A model file could only ever be linked to huggingface.co: `set_hf_url`
validated the URL with a huggingface-only regex, the agent fetched the card
from a hardcoded HF URL, and the readme processor built every relative image
path off `https://huggingface.co/{repo}/resolve/main`. ModelScope publishes the
same model-card convention (README.md + YAML frontmatter, often carrying
`base_model:` and `trigger_words:`) behind a public, key-less API, so the
enrichment pipeline could already serve it - it was the plumbing that was
HF-shaped, not the idea.

Make the external source a first-class, provider-driven concept:

- New `py/services/model_sources/` registry. A `ModelSource` owns URL
  recognition (lenient for stored values, strict for user input), the
  canonical page URL, model-card fetching, the asset base URL and the
  capability flags. `HuggingFaceSource` is the previous logic relocated;
  `ModelScopeSource` reads `/models/{o}/{n}/resolve/{master|main}/README.md`
  and falls back to `/api/v1/models/{o}/{n}/repo`. `TensorArtSource` is
  link-only on purpose: tensor.art answers plain HTTP clients with a
  Cloudflare challenge and its internal API (ap-east-1.tensorart.cloud /
  cn.tensorart.net) rejects every /v1/model/* route with "invalid
  authorization header", so it declares supports_enrichment=False rather than
  failing silently later.
- Metadata gains `source_platform` + `source_url`; `hf_url` stays as a
  read/write alias, written only for Hugging Face, so existing sidecars,
  cached rows and third-party consumers keep working. Normalisation runs at
  the scanner, the persistent cache (both directions, plus two new columns
  behind an ALTER migration) and the linking handler - which is what stops a
  user who switches sources from leaving a stale `hf_url` on a ModelScope
  model.
- The agent pipeline keys off the provider instead of `hf_url`: the fast-fail
  gate now explains *why* a model is skipped (no source / unknown source /
  source without a reachable card), the prompt context exposes
  source_url/source_id/source_label/asset_base_url while still filling the
  legacy hf_url/repo aliases, and the four README image extractors take a
  base_url (defaulting to HF) so relative paths resolve against the right
  site. Version grouping generalises to hf: / ms: / ta: keys.
- `POST /api/lm/set-hf-url` keeps its path and its legacy payload keys but
  accepts `source_url`, validates against every provider and returns the
  platform. `GET /api/lm/model-sources` lets the UI render the supported-site
  list from the server.
- Frontend: a `modelSourceHelpers` mirror of the registry drives the link
  dialog, the card/modal globe (branded "View on ModelScope/TensorArt"), the
  version-group key and the enrichment gate; the versions tab no longer sends
  ms:/ta: keys to the CivitAI API.

TensorArt stays in the list because provenance is worth keeping even when the
card is unreadable - the dialog says so plainly ("Sites that don't expose one
(currently TensorArt) can only be linked") and the context menu disables
enrichment with a matching tooltip, instead of the user getting
"Unsupported URL".

Verified against the real ModelScope API: jj3550945163/Krea-2-LORA returns a
1882-byte card whose frontmatter carries base_model/tags/trigger_words, and
relative images resolve to .../resolve/master/....

Tests: backend 2815 passed; frontend 1130 JS + 91 Vue passed; pytest
tests/i18n and a Jinja compile pass over templates/. The nine locales carry
[TODO: Translate] for the new strings, completed in the next commit.

py/services/base_model_service.py

@@ -21,6 +21,7 @@ from .model_query import (
     resolve_sub_type,
 )
 from .settings_manager import get_settings_manager
+from .model_sources import source_group_key
 from ..utils.civitai_utils import build_civitai_model_page_url
 
 logger = logging.getLogger(__name__)
@@ -742,29 +743,32 @@ class BaseModelService(ABC):
     @staticmethod
     def _extract_hf_group_key(item: Dict[str, Any]) -> Optional[str]:
         """Extract `hf:{owner}/{repo}` from item's ``hf_url``, or None."""
-        hf_url = item.get("hf_url") if isinstance(item, dict) else None
-        if not hf_url or not isinstance(hf_url, str):
-            return None
-        m = re.match(
-            r"https?://huggingface\.co/([^/]+/[^/]+)", hf_url.strip()
-        )
-        if not m:
-            return None
-        return f"hf:{m.group(1)}"
+        key = BaseModelService._extract_source_group_key(item)
+        return key if key and key.startswith("hf:") else None
+
+    @staticmethod
+    def _extract_source_group_key(item: Dict[str, Any]) -> Optional[str]:
+        """Return the external-source group key for *item*, or None.
+
+        Hugging Face keeps the historical ``hf:{owner}/{repo}`` shape; other
+        platforms use their own short prefix (``ms:`` / ``ta:``).
+        """
+        return source_group_key(item)
 
     @staticmethod
     def _extract_group_key(item: Dict[str, Any]) -> Union[int, str, None]:
-        """Return the group identity key: CivitAI modelId (int) or HF repo (str).
+        """Return the group identity key.
 
         Preference order:
         1. CivitAI ``modelId`` (int)
-        2. HF repo identity ``hf:{owner}/{repo}`` (str)
+        2. External model source identity, e.g. ``hf:{owner}/{repo}``,
+           ``ms:{owner}/{repo}``, ``ta:{model_id}`` (str)
         3. ``None`` (no known grouping source)
         """
         mid = BaseModelService._extract_model_id(item)
         if mid is not None:
             return mid
-        return BaseModelService._extract_hf_group_key(item)
+        return BaseModelService._extract_source_group_key(item)
 
     @staticmethod
     def _extract_model_id(item: Dict[str, Any]) -> Optional[int]: