fix(export): stop shipping project.time_active — it rewrote every shard of a project on every push

The app rewrites project.time_active continuously, and each shard embeds the
project row, so a 26-session project exported 26 shards per idle (5.9 MB) with
1-byte differences. The column is local activity bookkeeping: the receiving
machine keeps its own. Test asserts the shard is byte-identical when only
time_active changes. 128 green.

Author: aquelejota