ensure previews dir exists

when ffmpeg processes restart, there's a brief window where the preview frame generation pipeline is torn down and restarted. before these changes, ffmpeg only restarted on crash/stall recovery or full Frigate restart. Now that ffmpeg restarts happen on-demand via config changes, there's a higher chance a frontend request hits the preview_mp4 or preview_gif endpoints during that brief restart window when the directory might not exist yet. The existing os.listdir() call would throw FileNotFoundError without a directory existence check. this fix just checks if the directory exists and returns 404 if not, exactly how preview_thumbnail already handles the same scenario a few lines below

frigate/api/media.py

@@ -1263,6 +1263,13 @@ def preview_gif(
     else:
         # need to generate from existing images
         preview_dir = os.path.join(CACHE_DIR, "preview_frames")
+
+        if not os.path.isdir(preview_dir):
+            return JSONResponse(
+                content={"success": False, "message": "Preview not found"},
+                status_code=404,
+            )
+
         file_start = f"preview_{camera_name}"
         start_file = f"{file_start}-{start_ts}.{PREVIEW_FRAME_TYPE}"
         end_file = f"{file_start}-{end_ts}.{PREVIEW_FRAME_TYPE}"
@@ -1438,6 +1445,13 @@ def preview_mp4(
     else:
         # need to generate from existing images
         preview_dir = os.path.join(CACHE_DIR, "preview_frames")
+
+        if not os.path.isdir(preview_dir):
+            return JSONResponse(
+                content={"success": False, "message": "Preview not found"},
+                status_code=404,
+            )
+
         file_start = f"preview_{camera_name}"
         start_file = f"{file_start}-{start_ts}.{PREVIEW_FRAME_TYPE}"
         end_file = f"{file_start}-{end_ts}.{PREVIEW_FRAME_TYPE}"