transcode: add CBR/VBR/QP rate control + more x264 presets

Player/transcoding settings gain encoder controls:
- Rate-control mode VBR / CBR / QP (constant quantizer). x264: VBR uses
  -b:v/-maxrate/-bufsize, CBR adds -x264-params nal-hrd=cbr (min=max=b:v),
  QP uses -qp N (no bitrate). VP9 maps QP -> -crf N -b:v 0, CBR -> min=max=b:v.
- Preset list expanded to ultrafast/superfast/veryfast/faster/fast/medium
  (was only ultrafast/superfast); for VP9 the preset maps to cpu-used.
- New QP field in the UI, shown when rate control = QP, otherwise the
  bitrate field is shown.

Both /play.mp4 (archive) and /live.mp4 accept rc + qp query params.
All three modes verified producing valid output on the live restream.
VERSION -> 0.0.119.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api/recordings.py

@@ -9,7 +9,8 @@ from fastapi.responses import FileResponse, StreamingResponse
 
 from ..runtime import runtime
 from ..transcode import (H264_PROFILES, BITRATES, encode_args, norm_profile,
-                         norm_bitrate, norm_preset, norm_keyint, norm_codec)
+                         norm_bitrate, norm_preset, norm_keyint, norm_codec,
+                         norm_rc, norm_qp)
 
 router = APIRouter(prefix="/api/recordings", tags=["recordings"])
 
@@ -53,6 +54,8 @@ async def recording_transcoded(
     preset: str = Query(None),
     keyint: str = Query(None),
     codec: str = Query(None),
+    rc: str = Query(None),
+    qp: str = Query(None),
     start: float = Query(0.0),
 ):
     """Перекодирование H.265→H.264 на лету (fragmented mp4) — для браузеров без HEVC.
@@ -71,7 +74,8 @@ async def recording_transcoded(
         *(["-ss", f"{start:.3f}"] if start and start > 0 else []),
         "-i", rec.path,
         *encode_args(norm_codec(codec), norm_profile(profile), norm_bitrate(bitrate),
-                     preset=norm_preset(preset), keyint_sec=norm_keyint(keyint)),
+                     preset=norm_preset(preset), keyint_sec=norm_keyint(keyint),
+                     rc=norm_rc(rc), qp=norm_qp(qp)),
         "-an",
         "-movflags", "frag_keyframe+empty_moov+default_base_moof",
         "-f", "mp4", "pipe:1",