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/core/db/sync_db.rs

@@ -111,8 +111,8 @@ impl SyncDb {
                 "rule": "rule1",
                 "data": {
                     "text": segment.text,
-                    "start": segment.start,
-                    "end": segment.end,
+                    "start_time": segment.start_time,
+                    "end_time": segment.end_time,
                 },
             });
             let metadata = serde_json::json!({
@@ -132,8 +132,8 @@ impl SyncDb {
                 format!("{}", i),
                 ChunkType::Sentence,
                 ChunkRule::Rule1,
-                segment.start,
-                segment.end,
+                segment.start_time,
+                segment.end_time,
                 24.0, // fps
                 content,
             )