[Feature] Streaming playback from RSS audio URL

[lqdev]

Summary

Enable playing episodes directly from their RSS audio URL without downloading first. This adds a streaming mode alongside the existing download-then-play workflow.

Related: #131 (Meta-Epic: Cross-Platform Audio Playback)

Problem / Background

The MVP requires episodes to be downloaded (episode.local_path.is_some()) before playing. This means:

• Users must wait for full download before listening
• Storage space used for episodes they may only listen to once
• No "preview" functionality for deciding whether to subscribe

Every episode has an audio_url: String field (models.rs:65) that points to the media file.

Proposed Solution

1. HTTP Streaming Source

Create a custom rodio Source that reads from an HTTP stream:

pub struct HttpStreamSource {
    client: reqwest::Client,
    response: reqwest::Response,
    decoder: Decoder<BufReader<StreamReader>>,
    buffer: Vec<u8>,
    buffer_pos: usize,
}

2. Buffering Strategy

• Pre-buffer N seconds of audio before playback starts (configurable, default 5s)
• Continue buffering ahead while playing
• Handle network interruptions with retry logic
• Show buffer state in NowPlaying UI (buffer bar + playback bar)

3. PlaybackBackend Extension

Add to PlaybackBackend trait:

fn play_url(&mut self, url: &str) -> Result<(), AudioError>;
fn is_buffering(&self) -> bool;
fn buffer_progress(&self) -> Option<f64>; // 0.0 to 1.0

4. Cache While Streaming (Optional)

Write streamed bytes to disk as a side effect, so the episode is downloaded by the time playback finishes:

// Tee the stream: decode for playback + write to file

Files to Modify

File	Change
src/audio/mod.rs	Add play_url() to PlaybackBackend trait
src/audio/rodio_backend.rs	Implement HTTP streaming source
src/audio/http_stream.rs	New — HTTP streaming + buffering logic
src/ui/buffers/now_playing.rs	Show buffer progress indicator
src/ui/mod.rs	Add StreamEpisode UIAction variant

Acceptance Criteria

• Can play an episode from its audio_url without downloading
• Buffering indicator shows in NowPlaying buffer
• Handles network interruptions gracefully (retry, show error)
• Seek works within buffered range
• Optional: cache-while-streaming saves file to disk

Implementation Notes

• reqwest's .bytes_stream() returns impl Stream<Item = Result<Bytes>> — need to adapt to rodio's synchronous Source trait
• Consider using tokio::sync::mpsc to bridge async HTTP stream to sync audio thread
• Byte-range requests (Range: bytes=N-M) enable seeking in HTTP streams (if server supports it)
• This is XL effort — consider as a separate phase or even separate epic
• External player backend could implement streaming trivially: mpv <url> works out of the box