Switch data source to Finnhub API; remove synthetic data fallback

Adds FinnhubDataProvider as a drop-in replacement for ForexDataProvider.
Rate limit or API errors raise DataUnavailableError immediately — the
Telegram bot shows a clean per-TF error card rather than fake prices.

- data_provider_finnhub.py: Finnhub client, OANDA symbol mapping,
  4h resampled from 60-min candles, shared module-level rate limiter
  (60 calls/min free tier), full OHLCV validation copied from original
- signal_aggregator.py: import FinnhubDataProvider alias ForexDataProvider
- config.py: add finnhub_api_key field (FINNHUB_API_KEY env var overrides)
- telegram_bot.py: remove synthetic-data warning and _SUPPORTED_PAIRS;
  unsupported pairs now show the engine's "data unavailable" card
- requirements.txt: add finnhub-python>=1.0

Set FINNHUB_API_KEY env var before running. main branch is untouched.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

signal_aggregator.py

@@ -24,7 +24,7 @@ from config import (
     PlatformConfig, SignalStrength, Timeframe,
     SYMBOL_DISPLAY,
 )
-from data_provider import ForexDataProvider
+from data_provider_finnhub import FinnhubDataProvider as ForexDataProvider
 from engine import TechnicalEngine, TechnicalAnalysis
 from news_engine import NewsEngine, FundamentalContext