Cap uploads, GC empty boards, drop classic UI

Close residual audit P2: 5 MiB recording limit, opportunistic orphan
board GC, and remove static/index.html.

.env.example

@@ -11,3 +11,7 @@ STORK_DATA=./data
 # STORK_RL_BOARDS_PER_HOUR=5
 # STORK_RL_SESSIONS_PER_HOUR=60
 # STORK_RL_UPLOADS_PER_HOUR=30
+# Max recording upload size in bytes (default 5 MiB).
+# STORK_MAX_UPLOAD_BYTES=5242880
+# Delete empty boards older than this many hours (on create). Default 48.
+# STORK_ORPHAN_BOARD_HOURS=48