Fix the font_family setting and response encoding.

[touchao123]

When attempting to add Chinese subtitles to videos using the /v1/video/caption endpoint, I've discovered that specifying "font_family": "HarmonyOS Sans SC" within the JSON request results in the subtitles failing to render.
Two issues are present:

1. parameter transmission,
2. the download of captions is failing to utilize the correct encoding, leading to libass being unable to parse them and consequently falling back to Arial.ttf.
3. This patch rectifies these issues.

[HarrisonFisher]

@touchao123 The other changes you made, besides changing the encoding, do not appear to be needed

[touchao123]

@touchao123 The other changes you made, besides changing the encoding, do not appear to be needed
Correct, I've removed unnecessary lines. In the meantime, the practical running logs are attached here:

[Parsed_subtitles_0 @ 0x7336400024c0] Using font provider fontconfig
[Parsed_subtitles_0 @ 0x7336400024c0] fontselect: (HarmonyOS Sans SC, 400, 0) -> /usr/share/fonts/custom/HarmonyOS_Sans_SC_Regular.ttf, 0, HarmonyOS_Sans_SC
[libx264 @ 0x6246205bbb80] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2 AVX512