feat: Add external expert sources (Wikidata SPARQL + ConceptNet API)

- Add expert_cache.py: TTL-based in-memory cache (1 hour default)
- Add expert_source_service.py: WikidataProvider and ConceptNetProvider
  - Wikidata SPARQL queries for occupations with Chinese labels
  - ConceptNet API queries for occupation-related concepts
  - Random selection from cached pool
- Update schemas.py: Add ExpertSource enum (llm/wikidata/conceptnet)
- Update ExpertTransformationRequest with expert_source and expert_language
- Update router: Conditionally use external sources with LLM fallback
  - New SSE events: expert_source, expert_fallback
- Update frontend types with ExpertSource

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

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

backend/app/services/expert_cache.py

@@ -0,0 +1,92 @@
+"""Expert 資料快取模組
+
+提供 TTL-based 快取機制，減少外部 API 呼叫。
+"""
+
+import time
+from dataclasses import dataclass
+from typing import Dict, List, Optional
+
+
+@dataclass
+class CacheEntry:
+    """快取項目"""
+    data: List[dict]
+    timestamp: float
+
+
+class ExpertCache:
+    """TTL 快取，用於儲存外部來源的職業資料"""
+
+    def __init__(self, ttl_seconds: int = 3600):
+        """
+        初始化快取
+
+        Args:
+            ttl_seconds: 快取存活時間（預設 1 小時）
+        """
+        self._cache: Dict[str, CacheEntry] = {}
+        self._ttl = ttl_seconds
+
+    def get(self, key: str) -> Optional[List[dict]]:
+        """
+        取得快取資料
+
+        Args:
+            key: 快取鍵（如 "wikidata:zh:occupations"）
+
+        Returns:
+            快取的資料列表，若不存在或已過期則回傳 None
+        """
+        entry = self._cache.get(key)
+        if entry is None:
+            return None
+
+        # 檢查是否過期
+        if time.time() - entry.timestamp > self._ttl:
+            del self._cache[key]
+            return None
+
+        return entry.data
+
+    def set(self, key: str, data: List[dict]) -> None:
+        """
+        設定快取資料
+
+        Args:
+            key: 快取鍵
+            data: 要快取的資料列表
+        """
+        self._cache[key] = CacheEntry(
+            data=data,
+            timestamp=time.time()
+        )
+
+    def invalidate(self, key: Optional[str] = None) -> None:
+        """
+        清除快取
+
+        Args:
+            key: 要清除的鍵，若為 None 則清除全部
+        """
+        if key is None:
+            self._cache.clear()
+        elif key in self._cache:
+            del self._cache[key]
+
+    def get_stats(self) -> dict:
+        """取得快取統計資訊"""
+        now = time.time()
+        valid_count = sum(
+            1 for entry in self._cache.values()
+            if now - entry.timestamp <= self._ttl
+        )
+        return {
+            "total_entries": len(self._cache),
+            "valid_entries": valid_count,
+            "ttl_seconds": self._ttl
+        }
+
+
+# 全域快取實例
+expert_cache = ExpertCache()