Forex: convert to/from crypto via CoinGecko (keyless)

Wire a curated set of crypto assets (BTC, ETH, SOL, XRP, DOGE, ADA, LTC)
into the !fx conversion path only. fxLivePairRate now resolves each side
to units-per-USD and crosses them, so USD/fiat/crypto mix uniformly; fiat
sides still batch into one Frankfurter call, crypto hits CoinGecko's
keyless simple/price with a 60s cache.

Analysis (rate/report/setalert) stays fiat-only on purpose -- a daily
snapshot buy-signal/52w range is meaningless for 24/7 crypto.

internal/plugin/forex_crypto.go

@@ -0,0 +1,119 @@
+package plugin
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"strings"
+	"sync"
+	"time"
+)
+
+// ── Crypto support (CoinGecko, keyless) ──────────────────────────────────────
+//
+// CoinGecko's public simple/price endpoint needs no API key and no usage tier,
+// matching Frankfurter's keyless model. Crypto is wired into the *conversion*
+// path only (`!fx 100 USD to BTC`). The rate/report/alert analysis stays
+// fiat-only on purpose: a daily-snapshot "buy signal" or 52-week range is
+// meaningless for 24/7 crypto markets.
+
+type fxCryptoInfo struct {
+	CoinGeckoID string
+	Emoji       string
+	Label       string
+}
+
+// fxCryptoMeta maps a ticker to its CoinGecko coin ID and display metadata.
+// Add a row here to support another asset.
+var fxCryptoMeta = map[string]fxCryptoInfo{
+	"BTC":  {"bitcoin", "₿", "Bitcoin"},
+	"ETH":  {"ethereum", "Ξ", "Ethereum"},
+	"SOL":  {"solana", "◎", "Solana"},
+	"XRP":  {"ripple", "✕", "XRP"},
+	"DOGE": {"dogecoin", "🐕", "Dogecoin"},
+	"ADA":  {"cardano", "₳", "Cardano"},
+	"LTC":  {"litecoin", "Ł", "Litecoin"},
+}
+
+// fxIsCrypto reports whether sym is a supported crypto asset.
+func fxIsCrypto(sym string) bool {
+	_, ok := fxCryptoMeta[strings.ToUpper(sym)]
+	return ok
+}
+
+// fxIsConvertible reports whether a symbol can appear in a conversion —
+// USD, a tracked fiat currency, or a supported crypto asset.
+func fxIsConvertible(cur string) bool {
+	return fxIsSupported(cur) || fxIsCrypto(cur)
+}
+
+const coinGeckoBaseURL = "https://api.coingecko.com/api/v3"
+
+// Short-TTL price cache to stay well under CoinGecko's public rate limit and
+// keep repeated conversions snappy.
+const cgCacheTTL = 60 * time.Second
+
+var (
+	cgMu    sync.Mutex
+	cgCache = map[string]cgCacheEntry{}
+)
+
+type cgCacheEntry struct {
+	priceUSD float64
+	at       time.Time
+}
+
+// cgSpotUSD returns the USD spot price for one unit of the given crypto symbol.
+func (p *ForexPlugin) cgSpotUSD(sym string) (float64, error) {
+	sym = strings.ToUpper(sym)
+	info, ok := fxCryptoMeta[sym]
+	if !ok {
+		return 0, fmt.Errorf("unsupported crypto %s", sym)
+	}
+
+	cgMu.Lock()
+	if e, ok := cgCache[sym]; ok && time.Since(e.at) < cgCacheTTL {
+		cgMu.Unlock()
+		return e.priceUSD, nil
+	}
+	cgMu.Unlock()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 8*time.Second)
+	defer cancel()
+
+	url := fmt.Sprintf("%s/simple/price?ids=%s&vs_currencies=usd", coinGeckoBaseURL, info.CoinGeckoID)
+	req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
+	if err != nil {
+		return 0, err
+	}
+
+	resp, err := p.httpClient.Do(req)
+	if err != nil {
+		return 0, fmt.Errorf("coingecko API error: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return 0, fmt.Errorf("coingecko API returned %d", resp.StatusCode)
+	}
+
+	var data map[string]map[string]float64
+	if err := json.NewDecoder(resp.Body).Decode(&data); err != nil {
+		return 0, fmt.Errorf("coingecko decode error: %w", err)
+	}
+	entry, ok := data[info.CoinGeckoID]
+	if !ok {
+		return 0, fmt.Errorf("no price for %s", sym)
+	}
+	price, ok := entry["usd"]
+	if !ok || price <= 0 {
+		return 0, fmt.Errorf("invalid price for %s", sym)
+	}
+
+	cgMu.Lock()
+	cgCache[sym] = cgCacheEntry{priceUSD: price, at: time.Now()}
+	cgMu.Unlock()
+
+	return price, nil
+}