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/watchlist/commands.go

@@ -0,0 +1,163 @@
+package watchlist
+
+import (
+	"fmt"
+	"log/slog"
+	"strings"
+	"time"
+
+	"maunium.net/go/mautrix/id"
+)
+
+// DMSender is the interface for sending DMs to users.
+type DMSender interface {
+	SendDM(userID id.UserID, text string) error
+}
+
+// CommandHandler handles watchlist commands received via DM.
+type CommandHandler struct {
+	store  *Store
+	sender DMSender
+}
+
+// NewCommandHandler creates a new command handler.
+func NewCommandHandler(store *Store, sender DMSender) *CommandHandler {
+	return &CommandHandler{store: store, sender: sender}
+}
+
+// HandleMessage parses and dispatches a DM command.
+func (h *CommandHandler) HandleMessage(senderID, body string) {
+	body = strings.TrimSpace(body)
+	if body == "" {
+		return
+	}
+
+	var cmd, args string
+	if idx := strings.IndexByte(body, ' '); idx > 0 {
+		cmd = strings.ToLower(body[:idx])
+		args = strings.TrimSpace(body[idx+1:])
+	} else {
+		cmd = strings.ToLower(body)
+	}
+
+	uid := id.UserID(senderID)
+
+	switch cmd {
+	case "!watch":
+		h.handleWatch(uid, args)
+	case "!unwatch":
+		h.handleUnwatch(uid, args)
+	case "!extend":
+		h.handleExtend(uid, args)
+	case "!watchlist":
+		h.handleList(uid)
+	case "!help":
+		h.handleHelp(uid)
+	}
+}
+
+func (h *CommandHandler) handleWatch(userID id.UserID, gameName string) {
+	if gameName == "" {
+		h.reply(userID, "Usage: !watch <game name>")
+		return
+	}
+
+	added, err := h.store.AddWatch(string(userID), gameName)
+	if err != nil {
+		slog.Error("watchlist: add failed", "user", userID, "game", gameName, "error", err)
+		h.reply(userID, "Something went wrong. Please try again.")
+		return
+	}
+
+	if !added {
+		h.reply(userID, fmt.Sprintf("You're already watching \"%s\".", gameName))
+		return
+	}
+
+	expires := time.Now().Add(watchDuration)
+	h.reply(userID, fmt.Sprintf("Added \"%s\" to your watchlist. I'll DM you when a deal appears. Expires %s.",
+		gameName, expires.Format("January 2, 2006")))
+}
+
+func (h *CommandHandler) handleUnwatch(userID id.UserID, gameName string) {
+	if gameName == "" {
+		h.reply(userID, "Usage: !unwatch <game name>")
+		return
+	}
+
+	removed, err := h.store.RemoveWatch(string(userID), gameName)
+	if err != nil {
+		slog.Error("watchlist: remove failed", "user", userID, "game", gameName, "error", err)
+		h.reply(userID, "Something went wrong. Please try again.")
+		return
+	}
+
+	if !removed {
+		h.reply(userID, fmt.Sprintf("No watch found for \"%s\".", gameName))
+		return
+	}
+
+	h.reply(userID, fmt.Sprintf("Removed \"%s\" from your watchlist.", gameName))
+}
+
+func (h *CommandHandler) handleExtend(userID id.UserID, gameName string) {
+	if gameName == "" {
+		h.reply(userID, "Usage: !extend <game name>")
+		return
+	}
+
+	extended, err := h.store.ExtendWatch(string(userID), gameName)
+	if err != nil {
+		slog.Error("watchlist: extend failed", "user", userID, "game", gameName, "error", err)
+		h.reply(userID, "Something went wrong. Please try again.")
+		return
+	}
+
+	if !extended {
+		h.reply(userID, fmt.Sprintf("No watch found for \"%s\".", gameName))
+		return
+	}
+
+	expires := time.Now().Add(watchDuration)
+	h.reply(userID, fmt.Sprintf("Extended \"%s\" — now expires %s.",
+		gameName, expires.Format("January 2, 2006")))
+}
+
+func (h *CommandHandler) handleList(userID id.UserID) {
+	entries, err := h.store.ListWatches(string(userID))
+	if err != nil {
+		slog.Error("watchlist: list failed", "user", userID, "error", err)
+		h.reply(userID, "Something went wrong. Please try again.")
+		return
+	}
+
+	if len(entries) == 0 {
+		h.reply(userID, "Your watchlist is empty. Use !watch <game name> to add one.")
+		return
+	}
+
+	var sb strings.Builder
+	sb.WriteString(fmt.Sprintf("Your watchlist (%d):\n", len(entries)))
+	for _, e := range entries {
+		sb.WriteString(fmt.Sprintf("  - %s (expires %s)\n", e.GameName, e.ExpiresAt.Format("January 2, 2006")))
+	}
+	sb.WriteString("\nCommands: !watch, !unwatch, !extend, !watchlist")
+
+	h.reply(userID, sb.String())
+}
+
+func (h *CommandHandler) handleHelp(userID id.UserID) {
+	h.reply(userID, "Watchlist commands:\n"+
+		"  !watch <game name> — Watch for deals on a game\n"+
+		"  !unwatch <game name> — Remove a game from your watchlist\n"+
+		"  !extend <game name> — Reset the 180-day expiry timer\n"+
+		"  !watchlist — Show your current watches\n"+
+		"  !help — Show this message\n\n"+
+		"Watches expire after 180 days. You'll get a reminder 7 days before.")
+}
+
+func (h *CommandHandler) reply(userID id.UserID, text string) {
+	if err := h.sender.SendDM(userID, text); err != nil {
+		slog.Error("watchlist: failed to send DM", "user", userID, "error", err)
+	}
+}