refactor: split mutex, add download queue, wire context cancellation

- Split single sync.Mutex into per-map mutexes (activeJobs, stepStates,
  rateLimiter, pendingInput, cancelFuncs) to reduce contention
- Add download semaphore (channel-buffered, 3 concurrent) as worker pool
- Add cancelFuncs map with context cancellation wired into download
  goroutines for clean shutdown
- Replace playlist busy-poll (500ms sleep loop) with synchronous
  runDownload + Done channel for completion tracking
- Use full UUID for job IDs instead of 8-char prefix
- Download thumbnail URL to temp file before sending (InputFileUpload)
- Change MAX_FILE_SIZE_MB default from 2000 to 50
- Clean up cancel funcs after normal download completion

internal/handler/handler.go

@@ -36,18 +36,29 @@ type PendingInput struct {
 	Data      map[string]string
 }
 
+const maxConcurrentDownloads = 3
+
 type Handler struct {
 	Bot          *tgbot.Bot
 	Repo         repo.Repository
 	YtDlp        *dl.Client
 	AllowedUsers map[int64]bool
+
 	activeJobs   map[int64]*domain.DownloadJob
 	stepStates   map[int64]*stepState
 	lastMsgTime  map[int64]time.Time
 	pendingInput map[int64]*PendingInput
-	mu           sync.Mutex
-	downloadDir  string
-	maxFileSize  int64
+	cancelFuncs  map[int64]context.CancelFunc
+
+	activeJobsMu   sync.Mutex
+	stepStatesMu   sync.Mutex
+	rateLimiterMu  sync.Mutex
+	pendingInputMu sync.Mutex
+	cancelFuncsMu  sync.Mutex
+
+	downloadDir string
+	maxFileSize int64
+	downloadSem chan struct{}
 }
 
 func NewHandler(bot *tgbot.Bot, store repo.Repository, ytdlp *dl.Client, allowed map[int64]bool, downloadDir string, maxFileSize int64) *Handler {
@@ -60,8 +71,10 @@ func NewHandler(bot *tgbot.Bot, store repo.Repository, ytdlp *dl.Client, allowed
 		stepStates:   make(map[int64]*stepState),
 		lastMsgTime:  make(map[int64]time.Time),
 		pendingInput: make(map[int64]*PendingInput),
+		cancelFuncs:  make(map[int64]context.CancelFunc),
 		downloadDir:  downloadDir,
 		maxFileSize:  maxFileSize,
+		downloadSem:  make(chan struct{}, maxConcurrentDownloads),
 	}
 	go h.periodicCleanup()
 	return h
@@ -70,27 +83,29 @@ func NewHandler(bot *tgbot.Bot, store repo.Repository, ytdlp *dl.Client, allowed
 func (h *Handler) periodicCleanup() {
 	for {
 		time.Sleep(domain.CleanupInterval)
-		h.mu.Lock()
 
-		// Clean rate limit entries
-		cutoff := time.Now().Add(-domain.RateLimitInterval * 2)
-		for uid, t := range h.lastMsgTime {
-			if t.Before(cutoff) {
-				delete(h.lastMsgTime, uid)
+		func() {
+			h.rateLimiterMu.Lock()
+			defer h.rateLimiterMu.Unlock()
+			cutoff := time.Now().Add(-domain.RateLimitInterval * 2)
+			for uid, t := range h.lastMsgTime {
+				if t.Before(cutoff) {
+					delete(h.lastMsgTime, uid)
+				}
 			}
-		}
+		}()
 
-		// Clean stale pending inputs
-		pendingCutoff := time.Now().Add(-5 * time.Minute)
-		for chatID, pi := range h.pendingInput {
-			if pi.CreatedAt.Before(pendingCutoff) {
-				delete(h.pendingInput, chatID)
+		func() {
+			h.pendingInputMu.Lock()
+			defer h.pendingInputMu.Unlock()
+			pendingCutoff := time.Now().Add(-5 * time.Minute)
+			for chatID, pi := range h.pendingInput {
+				if pi.CreatedAt.Before(pendingCutoff) {
+					delete(h.pendingInput, chatID)
+				}
 			}
-		}
+		}()
 
-		h.mu.Unlock()
-
-		// Clean old temp files (>1h)
 		h.cleanOldTempFiles()
 	}
 }
@@ -116,8 +131,8 @@ func (h *Handler) cleanOldTempFiles() {
 }
 
 func (h *Handler) isRateLimited(userID int64) bool {
-	h.mu.Lock()
-	defer h.mu.Unlock()
+	h.rateLimiterMu.Lock()
+	defer h.rateLimiterMu.Unlock()
 	now := time.Now()
 	if last, ok := h.lastMsgTime[userID]; ok && now.Sub(last) < domain.RateLimitInterval {
 		return true
@@ -147,19 +162,19 @@ func (h *Handler) HandleMessage(ctx context.Context, msg *models.Message) {
 	slog.Info("message", "chat_id", msg.Chat.ID, "text", msg.Text)
 
 	if msg.Text[0] == '/' {
-		h.mu.Lock()
+		h.pendingInputMu.Lock()
 		delete(h.pendingInput, msg.Chat.ID)
-		h.mu.Unlock()
+		h.pendingInputMu.Unlock()
 		h.routeCommand(ctx, msg, userID)
 		return
 	}
 
-	h.mu.Lock()
+	h.pendingInputMu.Lock()
 	pi, hasPI := h.pendingInput[msg.Chat.ID]
 	if hasPI {
 		delete(h.pendingInput, msg.Chat.ID)
 	}
-	h.mu.Unlock()
+	h.pendingInputMu.Unlock()
 
 	if hasPI {
 		h.processPendingInput(ctx, msg, userID, pi)
@@ -212,9 +227,9 @@ func (h *Handler) HandleCallback(ctx context.Context, cb *models.CallbackQuery)
 	}
 	slog.Info("callback", "chat_id", chatID, "data", cb.Data)
 
-	h.mu.Lock()
+	h.pendingInputMu.Lock()
 	delete(h.pendingInput, chatID)
-	h.mu.Unlock()
+	h.pendingInputMu.Unlock()
 
 	msgID := cb.Message.Message.ID
 	data := cb.Data
@@ -294,7 +309,7 @@ func (h *Handler) routePrefixedCallback(ctx context.Context, chatID int64, msgID
 }
 
 func (h *Handler) setPending(ctx context.Context, chatID int64, msgID int, userID int64, kind PendingKind, data map[string]string) {
-	h.mu.Lock()
+	h.pendingInputMu.Lock()
 	h.pendingInput[chatID] = &PendingInput{
 		Kind:      kind,
 		UserID:    userID,
@@ -302,7 +317,7 @@ func (h *Handler) setPending(ctx context.Context, chatID int64, msgID int, userI
 		MsgID:     msgID,
 		Data:      data,
 	}
-	h.mu.Unlock()
+	h.pendingInputMu.Unlock()
 }
 
 func (h *Handler) processPendingInput(ctx context.Context, msg *models.Message, userID int64, pi *PendingInput) {
@@ -412,26 +427,40 @@ func (h *Handler) backToMenu(ctx context.Context, chatID int64, msgID int, cbID
 }
 
 func (h *Handler) clearActiveJob(userID int64) {
-	h.mu.Lock()
-	defer h.mu.Unlock()
-	if job, ok := h.activeJobs[userID]; ok {
+	h.activeJobsMu.Lock()
+	job, ok := h.activeJobs[userID]
+	if ok {
 		dl.CancelDownload(job)
 		delete(h.activeJobs, userID)
 	}
+	h.activeJobsMu.Unlock()
+
+	h.cancelFuncsMu.Lock()
+	if cancel, ok := h.cancelFuncs[userID]; ok {
+		cancel()
+		delete(h.cancelFuncs, userID)
+	}
+	h.cancelFuncsMu.Unlock()
 }
 
 func (h *Handler) getActiveJob(userID int64) *domain.DownloadJob {
-	h.mu.Lock()
-	defer h.mu.Unlock()
+	h.activeJobsMu.Lock()
+	defer h.activeJobsMu.Unlock()
 	return h.activeJobs[userID]
 }
 
 func (h *Handler) setActiveJob(userID int64, job *domain.DownloadJob) {
-	h.mu.Lock()
-	defer h.mu.Unlock()
+	h.activeJobsMu.Lock()
+	defer h.activeJobsMu.Unlock()
 	h.activeJobs[userID] = job
 }
 
+func (h *Handler) setCancelFunc(userID int64, cancel context.CancelFunc) {
+	h.cancelFuncsMu.Lock()
+	defer h.cancelFuncsMu.Unlock()
+	h.cancelFuncs[userID] = cancel
+}
+
 func formatBytes(bytes int64) string {
 	const unit = 1024
 	if bytes < unit {
@@ -455,7 +484,7 @@ func formatBytes(bytes int64) string {
 }
 
 func (h *Handler) generateJobID() string {
-	return uuid.NewString()[:8]
+	return uuid.NewString()
 }
 
 // handleStart shows the main menu and cleans up any stale state.