feat: ASR output frame numbers + rename start/end to start_time/end_time

- Python: asr_processor.py detects FPS from CUT/ffprobe (no fallback), outputs start_frame/end_frame
- Rust: All AsrSegment structs use start_time/end_time with #[serde(alias)] for backward compat
- store_asr_chunks: prefers ASR output frames, falls back to time-based conversion
- Added backward compatibility test for old JSON format (start/end)

Breaking change: ffprobe/CUT FPS failure now aborts instead of using default 24fps

src/worker/processor.rs

@@ -755,8 +755,11 @@ impl ProcessorPool {
             .iter()
             .enumerate()
             .map(|(i, segment)| {
-                let start_frame = (segment.start * fps).round() as i64;
-                let end_frame = (segment.end * fps).round() as i64;
+                // Prefer ASR output frames, fallback to time-based conversion
+                let start_frame = segment.start_frame
+                    .unwrap_or_else(|| (segment.start_time * fps).round() as i64);
+                let end_frame = segment.end_frame
+                    .unwrap_or_else(|| (segment.end_time * fps).round() as i64);
                 let data = serde_json::json!({
                     "text": segment.text,
                     "text_normalized": segment.text.to_lowercase(),
@@ -767,8 +770,8 @@ impl ProcessorPool {
                     i as i64,
                     start_frame,
                     end_frame,
-                    segment.start,
-                    segment.end,
+                    segment.start_time,
+                    segment.end_time,
                     data,
                 )
             })