feat(DATA-001): Add live GLD price feed service with Redis caching

- Create PriceFeed service using yfinance
- Cache prices in Redis with 60s TTL
- Add PriceData dataclass for type safety
- Support concurrent price fetching for multiple symbols

app/services/cache.py

@@ -70,3 +70,17 @@ class CacheService:
         if isinstance(value, datetime):
             return value.isoformat()
         raise TypeError(f"Object of type {type(value).__name__} is not JSON serializable")
+
+
+# Global cache instance
+_cache_instance: CacheService | None = None
+
+
+def get_cache() -> CacheService:
+    """Get or create global cache instance."""
+    global _cache_instance
+    if _cache_instance is None:
+        import os
+        redis_url = os.environ.get("REDIS_URL")
+        _cache_instance = CacheService(redis_url)
+    return _cache_instance