feat: Add curated expert occupations with local data sources

- Add curated occupations seed files (210 entries in zh/en) with specific domains
- Add DBpedia occupations data (2164 entries) for external source option
- Refactor expert_source_service to read from local JSON files
- Improve keyword generation prompts to leverage expert domain context
- Add architecture analysis documentation (ARCHITECTURE_ANALYSIS.md)
- Fix expert source selection bug (proper handling of empty custom_experts)
- Update frontend to support curated/dbpedia/wikidata expert sources

Key changes:
- backend/app/data/: Local occupation data files
- backend/app/services/expert_source_service.py: Simplified local file reading
- backend/app/prompts/expert_transformation_prompt.py: Better domain-aware prompts
- Removed expert_cache.py (no longer needed with local files)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/data/conceptnet_occupations_zh.json

@@ -0,0 +1,9 @@
+{
+  "metadata": {
+    "source": "conceptnet",
+    "language": "zh",
+    "fetched_at": "2025-12-04T07:26:26.994914+00:00",
+    "total_count": 0
+  },
+  "occupations": []
+}