Previously both backends defaulted to `CacheDir=tmp` and used the
same cache keys (`attendance_regular`, `attendance_juniors`,
`payments_transactions`, `exceptions_dict`) but stored different
shapes: Python caches post-processed view-model tuples
(e.g. `(members, sorted_months)`), Go caches raw sheet rows.
Whichever backend wrote last poisoned the cache for the other,
producing `ValueError: too many values to unpack (expected 2,
got 68)` on Python's /adults after the Go side populated the
file with 68 raw CSV rows.
This breaks the M5.4 `make parity` workflow that requires both
backends running side-by-side.
Fix: change Go's default to `tmp/go` so the two cache trees
never overlap. `CACHE_DIR` env var override still works.
`os.MkdirAll` already handles creating the new subdirectory on
first write.
Recovery for users with poisoned `tmp/`: hit /flush-cache on
the Python side once after pulling, then restart the Go server.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
