Provider abstraction: providers/ package with BaseDataProvider template

One class per provider (TwelveData, Oanda, Tiingo), all subclassing
BaseDataProvider which owns cache, validation/cleaning, freshness +
boundary guarantees, fetch_range pagination, and streaming stubs —
subclasses implement only _fetch_raw. config.json data_providers.active
selects the live feed; data_provider.py / oanda_provider.py stay as
back-compat shims. Module 16 provider-abstraction tests (253 total green).

Claude-Session: https://claude.ai/code/session_01Mc7XKYyPySVXiUpcN82QEK

pair_signals/__init__.py

@@ -12,7 +12,7 @@ from typing import Optional, List, Dict, Callable
 
 from config import PlatformConfig, Timeframe
 from signal_aggregator import SignalAggregator, SignalReport
-from data_provider import ForexDataProvider
+from providers import BaseDataProvider
 
 PAIR_PRODUCERS: Dict[str, Callable] = {}
 
@@ -28,7 +28,7 @@ def get_supported_pairs() -> List[str]:
 def produce_signal(
     symbol: str,
     primary_tf: Timeframe,
-    data_provider: ForexDataProvider,
+    data_provider: BaseDataProvider,
     confirmation_tfs: Optional[List[Timeframe]] = None,
 ) -> SignalReport:
     config_factory = PAIR_PRODUCERS.get(symbol)