Make encode bit-exact

This ensures only files that were actually changed have to be uploaded
on deploy.

encode.py

@@ -50,6 +50,7 @@ def encode_episode(podcast, episode, format):
         if format == "m4a":
             command.extend(["-c:a", "libfdk_aac"])
         command.extend(["-c:v", "copy"])
+        command.extend(["-bitexact"])  # deterministic output
         command.extend(options)
         for k, v in tags.items():
             command.extend(["-metadata", f"{k}={v}"])