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

oanda_daemon.py

@@ -46,7 +46,7 @@ from app_config import (
 from config import PAIR_INFO, PlatformConfig, Timeframe, XAUUSD
 from signal_aggregator import SignalAggregator, SignalReport, SignalStrength
 from pair_signals import PAIR_PRODUCERS
-from oanda_provider import OandaDataProvider
+from providers.oanda import OandaDataProvider
 from oanda_executor import OandaExecutor, OandaAPIError
 
 logger = logging.getLogger(__name__)