feat: add time-based throttling to progress broadcasts

Add 300ms minimum interval between progress broadcasts to reduce
WebSocket message volume. Broadcasts are sent immediately for
significant changes (>=1% or forced), otherwise throttled.

- Add MIN_BROADCAST_INTERVAL class constant (0.3s)
- Track last broadcast time per progress_id using time.monotonic()
- Clean up broadcast timestamps when progress completes/fails/cancels

src/server/services/progress_service.py

@@ -8,6 +8,7 @@ to connected clients.
 from __future__ import annotations
 
 import asyncio
+import time
 from dataclasses import dataclass, field
 from datetime import datetime, timezone
 from enum import Enum
@@ -168,6 +169,9 @@ class ProgressService:
     - Support for different progress types (download, scan, queue)
     """
 
+    # Minimum interval between broadcasts in seconds (300ms)
+    MIN_BROADCAST_INTERVAL: float = 0.3
+
     def __init__(self):
         """Initialize the progress service."""
         # Active progress operations: id -> ProgressUpdate
@@ -182,6 +186,9 @@ class ProgressService:
             str, List[Callable[[ProgressEvent], None]]
         ] = {}
 
+        # Track last broadcast time per progress_id for time-based throttling
+        self._last_broadcast_time: Dict[str, float] = {}
+
         # Lock for thread-safe operations
         self._lock = asyncio.Lock()
 
@@ -389,11 +396,21 @@ class ProgressService:
             update.status = ProgressStatus.IN_PROGRESS
             update.updated_at = datetime.now(timezone.utc)
 
-            # Only broadcast if significant change or forced
+            # Time-based throttle: broadcast at most every 300ms,
+            # or immediately for significant changes / forced broadcasts
+            now = time.monotonic()
+            last_broadcast = self._last_broadcast_time.get(progress_id, 0.0)
+            time_since_last = now - last_broadcast
             percent_change = abs(update.percent - old_percent)
-            should_broadcast = force_broadcast or percent_change >= 1.0
+
+            should_broadcast = (
+                force_broadcast
+                or percent_change >= 1.0
+                or time_since_last >= self.MIN_BROADCAST_INTERVAL
+            )
 
         if should_broadcast:
+            self._last_broadcast_time[progress_id] = time.monotonic()
             room = _get_room_for_progress_type(update.type)
             event = ProgressEvent(
                 event_type=f"{update.type.value}_progress",
@@ -442,6 +459,7 @@ class ProgressService:
 
             # Move to history
             del self._active_progress[progress_id]
+            self._last_broadcast_time.pop(progress_id, None)
             self._add_to_history(update)
 
             logger.info(
@@ -497,6 +515,7 @@ class ProgressService:
 
             # Move to history
             del self._active_progress[progress_id]
+            self._last_broadcast_time.pop(progress_id, None)
             self._add_to_history(update)
 
             logger.error(
@@ -548,6 +567,7 @@ class ProgressService:
 
             # Move to history
             del self._active_progress[progress_id]
+            self._last_broadcast_time.pop(progress_id, None)
             self._add_to_history(update)
 
             logger.info(