Data provider: key from config.json/env, provider registry, drop hardcoded key

Resolves the Twelve Data API key through app_config.provider_api_key()
(env → config.json → no default) instead of a hardcoded field on
PlatformConfig. Adds data_provider.PROVIDER_REGISTRY + make_data_provider()
so signal_aggregator and the backtester build the active provider through
one seam; telegram_bot.py gets the same one-line WS-key fix now (per Task 4's
already-planned edit for that line) so the suite doesn't crash on import
between this commit and Task 4's full wiring pass.

Claude-Session: https://claude.ai/code/session_01VPNF37MP9Kuv62B1VBrZ1b

backtest/gpu_backtest.py

@@ -55,7 +55,7 @@ from config import (  # noqa: E402
     Timeframe,
     XAUUSD,
 )
-from data_provider import ForexDataProvider  # noqa: E402
+from data_provider import ForexDataProvider, make_data_provider  # noqa: E402
 from engine import IndicatorSignal, TechnicalAnalysis, TechnicalEngine  # noqa: E402
 from signal_aggregator import SignalAggregator, SignalReport, TradeSetup  # noqa: E402
 
@@ -1679,7 +1679,7 @@ def run_backtest(
             f"vs PlatformConfig={platform_cfg.primary_timeframe.value}"
         )
 
-    provider = ForexDataProvider(platform_cfg)
+    provider = make_data_provider(platform_cfg)
 
     # ── Load all required timeframes ──
     timeframes = [bt_cfg.primary_tf] + list(bt_cfg.confirmation_tfs)