Rewrite Pastel from Python to Go with watchlist feature

Full rewrite of the gaming deals Matrix bot in Go for better performance
and a single static binary. Includes all original functionality (CheapShark,
ITAD, Epic free games, multi-currency pricing, dedup, preflight checks,
presence heartbeat) plus a new watchlist feature where users can DM the bot
to watch for specific game deals with 180-day auto-expiry.

New: systemd service file, DB migration script for Python-to-Go transition.

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

internal/currency/currency.go

@@ -0,0 +1,96 @@
+package currency
+
+import (
+	"encoding/json"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"strings"
+	"sync"
+	"time"
+)
+
+const (
+	apiURL   = "https://api.frankfurter.dev/v1/latest?base=USD&symbols=CAD,EUR,GBP"
+	cacheTTL = 12 * time.Hour
+)
+
+var symbols = map[string]string{
+	"USD": "$",
+	"CAD": "C$",
+	"EUR": "€",
+	"GBP": "£",
+}
+
+var currencies = []string{"CAD", "EUR", "GBP"}
+
+type Converter struct {
+	mu          sync.Mutex
+	rates       map[string]float64
+	lastFetched time.Time
+}
+
+func NewConverter() *Converter {
+	return &Converter{}
+}
+
+func (c *Converter) fetchRates() error {
+	resp, err := http.Get(apiURL)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("frankfurter API returned status %d", resp.StatusCode)
+	}
+
+	var result struct {
+		Rates map[string]float64 `json:"rates"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
+		return err
+	}
+
+	c.rates = result.Rates
+	c.lastFetched = time.Now()
+	return nil
+}
+
+// EnsureRates fetches exchange rates if the cache is stale or empty.
+func (c *Converter) EnsureRates() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if time.Since(c.lastFetched) < cacheTTL && c.rates != nil {
+		return
+	}
+
+	if err := c.fetchRates(); err != nil {
+		slog.Warn("failed to fetch exchange rates, using USD only", "error", err)
+	}
+}
+
+// FormatPrice converts a USD amount to multi-currency display string.
+func (c *Converter) FormatPrice(usd float64) string {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	parts := []string{fmt.Sprintf("$%.2f", usd)}
+
+	for _, cur := range currencies {
+		if rate, ok := c.rates[cur]; ok {
+			sym := symbols[cur]
+			parts = append(parts, fmt.Sprintf("%s%.2f", sym, usd*rate))
+		}
+	}
+
+	return strings.Join(parts, " · ")
+}
+
+// HasRates returns true if exchange rates are available.
+func (c *Converter) HasRates() bool {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.rates != nil && len(c.rates) > 0
+}