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/deals/cheapshark.go

@@ -0,0 +1,123 @@
+package deals
+
+import (
+	"encoding/json"
+	"fmt"
+	"log/slog"
+	"math"
+	"net/http"
+	"strconv"
+)
+
+var storeNames = map[string]string{
+	"1":  "Steam",
+	"7":  "GOG",
+	"11": "Humble Store",
+	"23": "GreenManGaming",
+}
+
+type CheapSharkDeal struct {
+	DealID       string
+	GameID       string
+	Title        string
+	SalePrice    float64
+	NormalPrice  float64
+	Savings      float64
+	DealRating   float64
+	StoreID      string
+	StoreName    string
+	LastChange   int64
+	SteamAppID   string
+	DealURL      string
+	IsHistLow    bool
+	DedupID      string
+}
+
+type cheapSharkRaw struct {
+	DealID     string `json:"dealID"`
+	GameID     string `json:"gameID"`
+	Title      string `json:"title"`
+	SalePrice  string `json:"salePrice"`
+	NormalPrice string `json:"normalPrice"`
+	Savings    string `json:"savings"`
+	DealRating string `json:"dealRating"`
+	StoreID    string `json:"storeID"`
+	LastChange int64  `json:"lastChange"`
+	SteamAppID string `json:"steamAppID"`
+}
+
+// FetchCheapSharkDeals fetches deals from CheapShark API.
+func FetchCheapSharkDeals(maxPrice float64, pageSize int) ([]CheapSharkDeal, error) {
+	url := fmt.Sprintf(
+		"https://www.cheapshark.com/api/1.0/deals?storeID=1,7,11,23&upperPrice=%d&sortBy=recent&desc=1&pageSize=%d",
+		int(maxPrice), pageSize,
+	)
+
+	resp, err := http.Get(url)
+	if err != nil {
+		return nil, fmt.Errorf("cheapshark request failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("cheapshark returned status %d", resp.StatusCode)
+	}
+
+	var raw []cheapSharkRaw
+	if err := json.NewDecoder(resp.Body).Decode(&raw); err != nil {
+		return nil, fmt.Errorf("cheapshark decode failed: %w", err)
+	}
+
+	var deals []CheapSharkDeal
+	for _, d := range raw {
+		sale, _ := strconv.ParseFloat(d.SalePrice, 64)
+		normal, _ := strconv.ParseFloat(d.NormalPrice, 64)
+		savings, _ := strconv.ParseFloat(d.Savings, 64)
+		rating, _ := strconv.ParseFloat(d.DealRating, 64)
+
+		storeName := storeNames[d.StoreID]
+		if storeName == "" {
+			storeName = "Unknown"
+		}
+
+		deals = append(deals, CheapSharkDeal{
+			DealID:      d.DealID,
+			GameID:      d.GameID,
+			Title:       d.Title,
+			SalePrice:   sale,
+			NormalPrice:  normal,
+			Savings:     savings,
+			DealRating:  rating,
+			StoreID:     d.StoreID,
+			StoreName:   storeName,
+			LastChange:  d.LastChange,
+			SteamAppID:  d.SteamAppID,
+			DealURL:     fmt.Sprintf("https://www.cheapshark.com/redirect?dealID=%s", d.DealID),
+			DedupID:     fmt.Sprintf("cheapshark-%s-%d", d.GameID, d.LastChange),
+		})
+	}
+
+	return deals, nil
+}
+
+// FilterCheapSharkDeals filters deals by rating, discount, and price thresholds.
+func FilterCheapSharkDeals(deals []CheapSharkDeal, minRating float64, minDiscount int, maxPrice float64) []CheapSharkDeal {
+	var filtered []CheapSharkDeal
+	for _, d := range deals {
+		discount := int(math.Floor(d.Savings))
+		if discount < minDiscount {
+			slog.Debug("cheapshark: skipping (low discount)", "title", d.Title, "discount", discount)
+			continue
+		}
+		if d.DealRating < minRating && d.DealRating != 0 {
+			slog.Debug("cheapshark: skipping (low rating)", "title", d.Title, "rating", d.DealRating)
+			continue
+		}
+		if d.SalePrice > maxPrice {
+			slog.Debug("cheapshark: skipping (too expensive)", "title", d.Title, "price", d.SalePrice)
+			continue
+		}
+		filtered = append(filtered, d)
+	}
+	return filtered
+}