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

signal_aggregator.py

@@ -15,7 +15,7 @@ import pandas as pd
 
 from chart_patterns import detect_chart_pattern
 from config import PlatformConfig, SignalStrength, Timeframe
-from data_provider import ForexDataProvider
+from data_provider import ForexDataProvider, make_data_provider
 from engine import TechnicalEngine, TechnicalAnalysis
 from news_engine import NewsEngine, FundamentalContext
 
@@ -104,7 +104,7 @@ class SignalAggregator:
 
     def __init__(self, config: PlatformConfig):
         self.config = config
-        self.data_provider = ForexDataProvider(config)
+        self.data_provider = make_data_provider(config)
         self.tech_engine = TechnicalEngine(config.technical)
         self._news_engine: Optional[NewsEngine] = None  # lazy