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

data_provider.py

@@ -5,7 +5,6 @@ Fetches and validates OHLCV via the official `twelvedata` Python SDK.
 """
 
 import logging
-import os
 import threading
 import time
 from collections import deque
@@ -14,6 +13,7 @@ from typing import Dict, Optional
 import pandas as pd
 from twelvedata import TDClient
 
+from app_config import AppConfigError, get_app_config, provider_api_key
 from config import PlatformConfig, Timeframe, XAUUSD, PAIR_INFO
 
 logger = logging.getLogger(__name__)
@@ -70,10 +70,7 @@ class ForexDataProvider:
 
     def __init__(self, config: PlatformConfig):
         self.config = config
-        self._api_key = (
-            os.environ.get("TWELVE_DATA_API_KEY")
-            or config.twelve_data_api_key
-        )
+        self._api_key = provider_api_key() or None
         self._cache: Dict[str, pd.DataFrame] = {}
         self._cache_timestamps: Dict[str, float] = {}
         # The bot shares one provider across a thread pool; cache eviction
@@ -264,8 +261,8 @@ class ForexDataProvider:
     ) -> pd.DataFrame:
         if self._td is None:
             raise DataIntegrityError(
-                "No Twelve Data API key. Set TWELVE_DATA_API_KEY env var "
-                "or config.twelve_data_api_key."
+                "No data-provider API key. Set the TWELVE_DATA_API_KEY env "
+                "var or data_providers.twelve_data.api_key in config.json."
             )
 
         td_symbol = self._to_td_symbol(symbol)
@@ -384,3 +381,20 @@ class ForexDataProvider:
             logger.info(f"{symbol}: Cleaned {cleaned}/{original_len} bars")
 
         return df
+
+
+# ── Provider registry ─────────────────────────────────────────────────
+# config.json data_providers.active selects from here. Adding a provider
+# later = one adapter class + one entry + one config block.
+PROVIDER_REGISTRY: Dict[str, type] = {"twelve_data": ForexDataProvider}
+
+
+def make_data_provider(config: PlatformConfig) -> ForexDataProvider:
+    """Build the ACTIVE data provider (config.json: data_providers.active)."""
+    name = get_app_config().active_provider
+    cls = PROVIDER_REGISTRY.get(name)
+    if cls is None:
+        raise AppConfigError(
+            f'Unknown data provider "{name}"; '
+            f"registered: {sorted(PROVIDER_REGISTRY)}")
+    return cls(config)