fix: wrong index for FPS replacement in preset-http-jpeg-generic

parse_preset_input() uses input[len(_user_agent_args) + 1] to find
the FPS placeholder, but preset-http-jpeg-generic does not include
_user_agent_args at the start of its list (only preset-http-mjpeg-generic
does). The FPS placeholder '{}' is at index 1, not index 3.

This means the detect_fps value overwrites '-1' (the stream_loop
argument) instead of the '{}' FPS placeholder, so the preset always
uses the literal string '{}' as the framerate.

frigate/ffmpeg_presets.py

@@ -436,7 +436,7 @@ def parse_preset_input(arg: Any, detect_fps: int) -> list[str]:
 
     if arg == "preset-http-jpeg-generic":
         input = PRESETS_INPUT[arg].copy()
-        input[len(_user_agent_args) + 1] = str(detect_fps)
+        input[1] = str(detect_fps)
         return input
 
     return PRESETS_INPUT.get(arg, None)