feat(17-04): implement /subscribe and /unsubscribe handlers

- handleSubscribe checks IsSubscribed, calls AddSubscriber with chat ID and username
- handleUnsubscribe calls RemoveSubscriber, reports rows affected
- Both use storage layer from Plan 17-02
- Removed stub implementations from bot.go

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/bot/subscribe.go

@@ -0,0 +1,59 @@
+package bot
+
+import (
+	"context"
+	"fmt"
+	"log"
+
+	"github.com/mymmrac/telego"
+)
+
+// handleSubscribe adds the requesting chat to the subscribers table.
+// If the chat is already subscribed, it informs the user without error.
+func (b *Bot) handleSubscribe(ctx context.Context, msg *telego.Message) {
+	chatID := msg.Chat.ID
+	var username string
+	if msg.From != nil {
+		username = msg.From.Username
+	}
+
+	subscribed, err := b.cfg.DB.IsSubscribed(chatID)
+	if err != nil {
+		log.Printf("subscribe: checking subscription for chat %d: %v", chatID, err)
+		_ = b.replyPlain(ctx, chatID, "Error checking subscription status. Please try again.")
+		return
+	}
+
+	if subscribed {
+		_ = b.replyPlain(ctx, chatID, "You are already subscribed to notifications.")
+		return
+	}
+
+	if err := b.cfg.DB.AddSubscriber(chatID, username); err != nil {
+		log.Printf("subscribe: adding subscriber chat %d: %v", chatID, err)
+		_ = b.replyPlain(ctx, chatID, fmt.Sprintf("Error subscribing: %v", err))
+		return
+	}
+
+	_ = b.replyPlain(ctx, chatID, "Subscribed! You will receive notifications when new API keys are found.")
+}
+
+// handleUnsubscribe removes the requesting chat from the subscribers table.
+// If the chat was not subscribed, it informs the user without error.
+func (b *Bot) handleUnsubscribe(ctx context.Context, msg *telego.Message) {
+	chatID := msg.Chat.ID
+
+	rows, err := b.cfg.DB.RemoveSubscriber(chatID)
+	if err != nil {
+		log.Printf("unsubscribe: removing subscriber chat %d: %v", chatID, err)
+		_ = b.replyPlain(ctx, chatID, fmt.Sprintf("Error unsubscribing: %v", err))
+		return
+	}
+
+	if rows == 0 {
+		_ = b.replyPlain(ctx, chatID, "You are not subscribed.")
+		return
+	}
+
+	_ = b.replyPlain(ctx, chatID, "Unsubscribed. You will no longer receive notifications.")
+}