feat: KuCoin provider — crypto OHLCV, no key, 1500 bars, all intervals

[FaustoS88]

Adds KuCoinProvider as a fourth free crypto data source.

What's added:

• src/ohlcv_hub/providers/kucoin.py — implements OHLCVProvider ABC
• Symbol normalisation: BTCUSDT → BTC-USDT (KuCoin's BASE-QUOTE format)
• Handles KuCoin's reverse-chronological response (newest-first → reversed to oldest-first)
• Candle field mapping: KuCoin row is [ts, open, close, high, low, volume, turnover] — close at index 2, high at 3
• All standard intervals: 1m, 5m, 15m, 30m, 1h, 4h, 1d, 1w
• No API key required
• Wired into crypto fallback chain: Binance → CoinGecko → Kraken → KuCoin → yfinance

Tests (tests/test_kucoin.py):

• 9 normalisation cases
• 9 supports() cases (crypto True, stock/forex/intl False)
• fetch() happy path: candle values, reversal, limit, all intervals, symbol format
• fetch() error cases: unsupported interval, HTTP error, API error code, empty data, network error, malformed rows

30 tests. 212 total passing. ruff clean.