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

telegram_bot.py

@@ -65,6 +65,7 @@ from config import (
     PlatformConfig, SignalStrength, Timeframe,
     XAUUSD, PAIR_INFO,
 )
+from app_config import provider_api_key
 from data_provider import ForexDataProvider
 from pair_signals import PAIR_PRODUCERS
 from signal_aggregator import SignalAggregator, SignalReport
@@ -609,7 +610,7 @@ _last_price_ts: Dict[str, float] = {}
 _scan_cooldowns: Dict[str, float] = {}
 
 _cfg = PlatformConfig()
-_WS_API_KEY = os.environ.get("TWELVE_DATA_API_KEY") or _cfg.twelve_data_api_key
+_WS_API_KEY = provider_api_key()
 _WS_URL = f"wss://ws.twelvedata.com/v1/quotes/price?apikey={_WS_API_KEY}"
 _WS_COOLDOWN = _cfg.ws_cooldown_seconds