fix(tts): improve Opus encoding quality for voice messages

Salvaged from PR #18861 (@shellybotmoyer). Switch the central Opus
transcoders from CBR 64k (-vbr off) to VBR 48k with -application voip and
-compression_level 10. The old CBR settings produced lower-quality speech
audio and occasionally failed to trigger Telegram's native voice-bubble
rendering. Applied to _ffmpeg_transcode_to_opus (the single transcoder
behind _convert_to_opus and _repair_ogg_container), the Gemini WAV→OGG
path, and the Matrix adapter boundary transcoder added in this branch.

Fixes #18818

plugins/platforms/matrix/adapter.py

@@ -249,7 +249,13 @@ def _matrix_transcode_voice_to_ogg(path: str) -> Optional[str]:
                 "-ac",
                 "1",
                 "-b:a",
-                "64k",
+                "48k",
+                "-vbr",
+                "on",
+                "-application",
+                "voip",
+                "-compression_level",
+                "10",
                 ogg_path,
             ],
             capture_output=True,

tools/tts_tool.py

@@ -1083,7 +1083,8 @@ def _ffmpeg_transcode_to_opus(input_path: str, ogg_path: str) -> Optional[str]:
     try:
         result = subprocess.run(
             ["ffmpeg", "-i", input_path, "-acodec", "libopus",
-             "-ac", "1", "-b:a", "64k", "-vbr", "off", "-f", "ogg",
+             "-ac", "1", "-b:a", "48k", "-vbr", "on",
+             "-application", "voip", "-compression_level", "10", "-f", "ogg",
              work_path, "-y"],
             capture_output=True, timeout=30,
             stdin=subprocess.DEVNULL,
@@ -2202,7 +2203,8 @@ def _generate_gemini_tts(text: str, output_path: str, tts_config: Dict[str, Any]
                 cmd = [
                     ffmpeg, "-i", wav_path,
                     "-acodec", "libopus", "-ac", "1",
-                    "-b:a", "64k", "-vbr", "off",
+                    "-b:a", "48k", "-vbr", "on",
+                    "-application", "voip", "-compression_level", "10",
                     "-y", "-loglevel", "error",
                     output_path,
                 ]