feat(F2): integrate AES-256-CTR encryption into AdaptiveRecorder

After FFmpeg finishes writing, _stop_ffmpeg() now reads VIGILAR_ENCRYPTION_KEY
and encrypts the MP4 to .vge format via encrypt_file(), updating the returned
RecordingSegment to reflect the encrypted file path and size.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

vigilar/camera/recorder.py

@@ -156,6 +156,27 @@ class AdaptiveRecorder:
             duration, self._frame_count, file_size,
         )
 
+        # Encrypt if key is available
+        encryption_key = os.environ.get("VIGILAR_ENCRYPTION_KEY")
+        if encryption_key and self._current_path and self._current_path.exists():
+            try:
+                from vigilar.storage.encryption import encrypt_file
+                vge_path = encrypt_file(str(self._current_path), encryption_key)
+                segment = RecordingSegment(
+                    file_path=vge_path,
+                    started_at=self._started_at,
+                    ended_at=ended_at,
+                    duration_s=duration,
+                    file_size=Path(vge_path).stat().st_size,
+                    trigger=self._current_trigger,
+                    fps=self._current_fps,
+                    frame_count=self._frame_count,
+                )
+                log.info("Encrypted recording: %s", Path(vge_path).name)
+            except Exception:
+                log.exception("Encryption failed for %s, keeping plain MP4",
+                              self._current_path.name)
+
         self._process = None
         self._current_path = None
         return segment