feat: initial implementation of uptodownbot

Telegram bot for downloading media from any site using yt-dlp.
Supports audio/video/both, quality selection, language picker,
container format selection, playlist pagination, progress updates,
per-user quotas, user preferences, download history, cancel at any step,
polling and webhook modes, proxy support, and SQLite persistence.

internal/domain/constants.go

@@ -0,0 +1,22 @@
+package domain
+
+import "time"
+
+const (
+	RateLimitInterval      = 1 * time.Second
+	ProgressThreshold      = 5.0
+	MaxFileSizeEnvKey      = "MAX_FILE_SIZE_MB"
+	DefaultMaxFileSize     = 2000
+	DefaultDailyQuotaMB    = 100
+	DefaultWeeklyQuotaMB   = 500
+	DefaultMonthlyQuotaMB  = 2000
+	CleanupInterval        = 10 * time.Second
+	FileRetention          = 1 * time.Hour
+	DownloadDirEnvKey      = "DOWNLOAD_DIR"
+	DefaultDownloadDir     = "/tmp/uptodown"
+	CookiesFileEnvKey      = "COOKIES_FILE"
+	DBPathEnvKey           = "DB_PATH"
+	DefaultDBPath          = "data/uptodown.db"
+	DateLayout             = "2006-01-02"
+	QuotaProgressThreshold = 50.0
+)