feat(17-02): add gocron dependency, subscribers and scheduled_jobs tables with CRUD

- Add gocron/v2 v2.19.1 as direct dependency
- Append subscribers and scheduled_jobs CREATE TABLE to schema.sql
- Implement full subscriber CRUD (Add/Remove/List/IsSubscribed)
- Implement full scheduled job CRUD (Save/List/Get/Delete/UpdateLastRun/SetEnabled)

pkg/storage/subscribers.go

@@ -0,0 +1,54 @@
+package storage
+
+import "time"
+
+// Subscriber represents a Telegram chat subscribed to scan notifications.
+type Subscriber struct {
+	ChatID       int64
+	Username     string
+	SubscribedAt time.Time
+}
+
+// AddSubscriber inserts or replaces a subscriber in the database.
+func (db *DB) AddSubscriber(chatID int64, username string) error {
+	_, err := db.sql.Exec(
+		`INSERT OR REPLACE INTO subscribers (chat_id, username, subscribed_at) VALUES (?, ?, CURRENT_TIMESTAMP)`,
+		chatID, username,
+	)
+	return err
+}
+
+// RemoveSubscriber deletes a subscriber by chat ID. Returns rows affected.
+func (db *DB) RemoveSubscriber(chatID int64) (int64, error) {
+	res, err := db.sql.Exec(`DELETE FROM subscribers WHERE chat_id = ?`, chatID)
+	if err != nil {
+		return 0, err
+	}
+	return res.RowsAffected()
+}
+
+// ListSubscribers returns all subscribers ordered by subscription time.
+func (db *DB) ListSubscribers() ([]Subscriber, error) {
+	rows, err := db.sql.Query(`SELECT chat_id, username, subscribed_at FROM subscribers ORDER BY subscribed_at`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var subs []Subscriber
+	for rows.Next() {
+		var s Subscriber
+		if err := rows.Scan(&s.ChatID, &s.Username, &s.SubscribedAt); err != nil {
+			return nil, err
+		}
+		subs = append(subs, s)
+	}
+	return subs, rows.Err()
+}
+
+// IsSubscribed returns true if the given chat ID is subscribed.
+func (db *DB) IsSubscribed(chatID int64) (bool, error) {
+	var count int
+	err := db.sql.QueryRow(`SELECT COUNT(*) FROM subscribers WHERE chat_id = ?`, chatID).Scan(&count)
+	return count > 0, err
+}