feat(modelscope): read the model-detail API for card extras

ModelScope's model card is not just README.md: the author's summary
(Description), the site-curated tags (OfficialTags), the internal
architecture enums (VisionFoundation/SubVisionFoundation) and — per
published version — the model filenames with that file's example images
(coverImages) and trigger words all live in the model-detail API.
AIGC repositories there frequently ship an auto-generated boilerplate
README and put the only useful text in Description, so reading just the
README yielded almost nothing.

Add `ModelSource.fetch_model_card_context()` returning a new
`ModelCardContext`, implemented by ModelScopeSource against the public
(no API key) detail endpoint. Example images are matched to the model's
basename through each version's `stats.fileList`, so every checkpoint in
a collection repository gets its own images rather than a sibling's.

Consume the context in the post-processor:

* example images seed `civitai.images` and, being per-file, take priority
  in the preview fallback chain
* the author summary becomes a paragraph in `modelDescription` and fills
  `civitai.description` when the LLM returns no short description
* site-curated tags are always merged in, which also fixes the official
  `character-enhancement` being dropped by the prompt's no-hyphen rule
* per-file trigger words are used before the repo-wide YAML
  `instance_prompt`
* an explicitly stated strength range is recovered by regex so
  `usage_tips` is populated even without an LLM

The prompt gains a Site-Provided Metadata section so the LLM can prefer
the site's first-hand data over its own guesses.

py/services/model_sources/__init__.py

@@ -11,6 +11,7 @@ from __future__ import annotations
 from .base import (
     GROUP_PREFIXES,
     HTTP_TIMEOUT,
+    ModelCardContext,
     ModelSource,
     ModelSourceError,
     SourceRef,
@@ -45,6 +46,7 @@ __all__ = [
     "GROUP_PREFIXES",
     "HTTP_TIMEOUT",
     "LEGACY_HF_URL_FIELD",
+    "ModelCardContext",
     "ModelSource",
     "ModelSourceError",
     "HuggingFaceSource",