refactor: Remove dead FFmpeg code from pmoparadise

Remove unused FFmpeg-based progressive streaming implementation that was
never completed and is not used anywhere in the codebase.

Changes:
- Delete src/ffmpeg_streaming.rs (173 lines of unfinished code with TODOs)
- Remove ffmpeg module import from lib.rs
- Remove ffmpeg feature from Cargo.toml
- Remove ffmpeg-next dependency from Cargo.toml

The current implementation uses claxon (StreamingPCMDecoder) and symphonia
(decode_block_audio) for FLAC decoding, which are fully functional.

Verified: cargo check passes successfully after removal.

Cargo.lock

@@ -1095,31 +1095,6 @@ dependencies = [
  "simd-adler32",
 ]
 
-[[package]]
-name = "ffmpeg-next"
-version = "8.0.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d658424d233cbd993a972dd73a66ca733acd12a494c68995c9ac32ae1fe65b40"
-dependencies = [
- "bitflags 2.10.0",
- "ffmpeg-sys-next",
- "libc",
-]
-
-[[package]]
-name = "ffmpeg-sys-next"
-version = "8.0.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9bca20aa4ee774fe384c2490096c122b0b23cf524a9910add0686691003d797b"
-dependencies = [
- "bindgen",
- "cc",
- "libc",
- "num_cpus",
- "pkg-config",
- "vcpkg",
-]
-
 [[package]]
 name = "find-msvc-tools"
 version = "0.1.4"
@@ -2905,7 +2880,6 @@ dependencies = [
  "bytes",
  "chrono",
  "claxon",
- "ffmpeg-next",
  "flacenc",
  "futures",
  "futures-util",