add ffmpeg feature

Cargo.toml

@@ -14,7 +14,7 @@ candle-flash-attn = { git = "https://github.com/huggingface/candle.git", version
 serde = "1.0.226"
 serde_json = "1.0.145"
 anyhow = "1.0.100"
-ffmpeg-next = "8.0.0"
+ffmpeg-next = { version = "8.0.0", optional = true }
 image = "0.25.8"
 reqwest = { version = "0.12.23", features = ["blocking"] }
 base64 = "0.22.1"
@@ -34,6 +34,7 @@ dirs = "6.0.0"
 [features]
 flash-attn=["candle-flash-attn"]
 cuda=["candle-nn/cuda", "candle-core/cuda", "candle-transformers/cuda"]
+ffmpeg=["ffmpeg-next"]
 
 [lints.clippy]
 needless_range_loop = "allow"