Install ffmpeg on all CI runners for sidecar build

pyannote.audio requires ffmpeg at import time (torchcodec loads
FFmpeg shared libraries). Install via brew (macOS), apt (Linux),
choco (Windows) before building the sidecar.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -54,6 +54,19 @@ jobs:
             echo "$env:USERPROFILE\.local\bin" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
           }
 
+      - name: Install ffmpeg (macOS)
+        if: matrix.platform == 'macos'
+        run: brew install ffmpeg
+
+      - name: Install ffmpeg (Linux)
+        if: matrix.platform == 'linux'
+        run: sudo apt-get update && sudo apt-get install -y ffmpeg
+
+      - name: Install ffmpeg (Windows)
+        if: matrix.platform == 'windows'
+        shell: powershell
+        run: choco install ffmpeg -y
+
       - name: Set up Python (Unix)
         if: matrix.platform != 'windows'
         run: uv python install ${{ env.PYTHON_VERSION }}