refactor(metadata): remove vestigial top-level trainedWords field

The field dates back to a development-stage bug in the enrich-metadata
(agent) pipeline, which briefly wrote trigger words at the top level of
model metadata instead of the established civitai.trainedWords location.
The write path was fixed before the feature merged to main (PR #1013)
and never shipped in any release, so no writer has existed since.

Remove the leftover pieces:

- BaseModelMetadata.trainedWords field (py/utils/models.py); sidecars
  from that dev window now pass the key through _unknown_fields instead
- HF download handler's strip-empty-trainedWords special case, reverting
  to saving the metadata object directly (py/routes/handlers/hf_handlers.py)
- trainedWords in the LLM enrichment context (agent_service.py)
- matching fallbacks/fixtures in the enrich_hf_validation harness and
  post-processor test

Trigger words continue to live in civitai.trainedWords for all model
sources, which is what the UI, agent post-processor, and metadata sync
all read and write.

tests/enrich_hf_validation/evaluation_engine.py

@@ -100,7 +100,7 @@ def evaluate_model(
     flagged issues.
     """
     civitai = metadata.get("civitai") or {}
-    trained_words: List[str] = civitai.get("trainedWords") or metadata.get("trainedWords") or []
+    trained_words: List[str] = civitai.get("trainedWords") or []
     short_desc: str = civitai.get("description") or ""
     tags: List[str] = metadata.get("tags") or []
     notes: str = metadata.get("notes") or ""