Provide 0.3s buffer for silence trimming

We want to trim the silence from recordings but risk cutting off
the first and last syllables, particularly if they are softer
sounds. This adds a reasonable length buffer for data collection.
During post-processing the sounds can be further trimmed if desired.

backend/app/audio.py

@@ -13,7 +13,10 @@ class Audio:
         while sound[trim_ms:trim_ms + Audio.chunk_size].dBFS \
                 < Audio.silence_threshold and trim_ms < len(sound):
             trim_ms += Audio.chunk_size
-
+        trim_buffer = 300  # buffer to prevent first sound getting cut
+        trim_ms -= trim_buffer
+        if trim_ms < 0:
+            trim_ms = 0
         return trim_ms
 
     @staticmethod