fix: validate pause points and frame extraction

- Get video duration BEFORE the render loop (not after)
- Clamp pause_point to 100ms before video end if it exceeds duration
- Add validation in _extract_frame() to verify frame was created
- Add debug logging for frame extraction timestamps

This prevents "Frame file not found" errors when pause points
calculated by Whisper refinement exceed the source video duration.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/video_renderer.py

@@ -307,7 +307,8 @@ class VideoRendererService:
 
             # Get detailed video AND audio properties for uniform encoding
             video_props = await self._get_video_properties(source_video_path)
-            logger.info(f"Source Properties: {video_props}")
+            source_duration = await self._get_video_duration(source_video_path)
+            logger.info(f"Source Properties: {video_props}, Duration: {source_duration:.2f}s")
 
             segment_files = []
             current_time = 0.0
@@ -320,6 +321,18 @@ class VideoRendererService:
                 cue_index = placement["ad_cue_index"]
                 ad_duration = placement["ad_duration"]
                 resume_from = placement.get("resume_from", pause_point)  # Fallback for backwards compat
+
+                # Validate pause_point is within video bounds
+                if pause_point >= source_duration:
+                    logger.warning(
+                        f"Cue {cue_index}: pause_point {pause_point:.2f}s exceeds video duration "
+                        f"{source_duration:.2f}s, clamping to {source_duration - 0.1:.2f}s"
+                    )
+                    pause_point = max(0, source_duration - 0.1)  # Clamp to 100ms before end
+
+                if resume_from >= source_duration:
+                    resume_from = pause_point
+
                 overlap = max(0, pause_point - resume_from)
 
                 # Get the AD audio for this cue
@@ -401,7 +414,6 @@ class VideoRendererService:
                 current_time = pause_point
 
             # 4. Add final segment from last pause point to end (re-encoded for uniformity)
-            source_duration = await self._get_video_duration(source_video_path)
             logger.info(
                 f"Final segment check: current_time={current_time:.2f}s, "
                 f"source_duration={source_duration:.2f}s, "
@@ -550,6 +562,8 @@ class VideoRendererService:
 
     async def _extract_frame(self, video_path: str, time_point: float, output_path: str):
         """Extract a single frame as PNG using ffmpeg."""
+        logger.debug(f"Extracting frame at {time_point:.2f}s from {video_path}")
+
         cmd = [
             self.ffmpeg_path,
             "-y",
@@ -561,6 +575,14 @@ class VideoRendererService:
         ]
         await self._run_ffmpeg(cmd)
 
+        # Verify frame was actually created (FFmpeg can "succeed" but produce nothing
+        # if time_point is beyond video duration)
+        if not os.path.exists(output_path):
+            raise FileNotFoundError(
+                f"Frame extraction failed: no frame created at {time_point:.2f}s "
+                f"(time may be beyond video duration)"
+            )
+
     async def _create_freeze_segment(
         self,
         frame_path: str,