Swap raw requests for Twelve Data Python SDK

data_provider now uses TDClient.time_series().as_pandas() instead of
hand-rolled REST calls. Throttle, validation, and parquet caching are
unchanged. Adds twelvedata>=1.2.0 to requirements.txt.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

requirements.txt

@@ -3,3 +3,4 @@ pandas>=2.0
 requests>=2.28
 pyarrow>=14.0
 tqdm>=4.66
+twelvedata>=1.2.0