fix: Clean retrigger preserves transcript_segments (pipeline input data)

Deleting transcript_segments left the pipeline with nothing to process —
all stages would skip immediately. Segments come from the ingest step,
not from pipeline stages 2-6. Only pipeline_events and key_moments
(pipeline output) are deleted during clean reprocess.

backend/routers/pipeline.py

@@ -189,10 +189,10 @@ async def clean_retrigger_pipeline(
     await db.execute(
         KeyMoment.__table__.delete().where(KeyMoment.source_video_id == video_id)
     )
-    # Delete transcript segments
-    await db.execute(
-        TranscriptSegment.__table__.delete().where(TranscriptSegment.source_video_id == video_id)
-    )
+    # Note: transcript_segments are NOT deleted — they are the pipeline's input
+    # data created during ingest, not pipeline output. Deleting them would leave
+    # the pipeline with nothing to process.
+    
     # Reset status
     video.processing_status = ProcessingStatus.not_started
     await db.commit()
@@ -200,7 +200,6 @@ async def clean_retrigger_pipeline(
     deleted_counts = {
         "pipeline_events": "cleared",
         "key_moments": "cleared",
-        "transcript_segments": "cleared",
     }
 
     # Best-effort Qdrant cleanup (non-blocking)