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

config.py

@@ -343,8 +343,6 @@ class PlatformConfig:
 
     data_lookback_days: int = 60
 
-    twelve_data_api_key: str = "f563b22ffdfd4924955decbe1a8c8701"
-
     refresh_interval: int = 300
 
     scan_pairs: List[str] = field(