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/model_sources/__init__.py

@@ -0,0 +1,61 @@
+"""External model-source providers (Hugging Face, ModelScope, TensorArt).
+
+This package is the single abstraction over "a site that hosts models and
+a model card".  See :mod:`py.services.model_sources.base` for the provider
+protocol and :mod:`py.services.model_sources.registry` for the lookup and
+metadata-normalisation helpers used across the codebase.
+"""
+
+from __future__ import annotations
+
+from .base import (
+    GROUP_PREFIXES,
+    HTTP_TIMEOUT,
+    ModelSource,
+    SourceRef,
+    USER_AGENT,
+    clean_source_url,
+    fetch_text,
+)
+from .huggingface import HuggingFaceSource
+from .modelscope import ModelScopeSource
+from .registry import (
+    LEGACY_HF_URL_FIELD,
+    SOURCE_PLATFORM_FIELD,
+    SOURCE_URL_FIELD,
+    detect_source,
+    get_source,
+    get_source_platform,
+    has_external_source,
+    list_sources,
+    normalize_metadata_source,
+    resolve_source_ref,
+    source_group_key,
+    source_label,
+)
+from .tensorart import TensorArtSource
+
+__all__ = [
+    "GROUP_PREFIXES",
+    "HTTP_TIMEOUT",
+    "LEGACY_HF_URL_FIELD",
+    "ModelSource",
+    "HuggingFaceSource",
+    "ModelScopeSource",
+    "SOURCE_PLATFORM_FIELD",
+    "SOURCE_URL_FIELD",
+    "SourceRef",
+    "TensorArtSource",
+    "USER_AGENT",
+    "clean_source_url",
+    "detect_source",
+    "fetch_text",
+    "get_source",
+    "get_source_platform",
+    "has_external_source",
+    "list_sources",
+    "normalize_metadata_source",
+    "resolve_source_ref",
+    "source_group_key",
+    "source_label",
+]