fix(go): default CacheDir to tmp/go to avoid Python collision

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>

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## 2026-05-07 23:05 CEST — fix(go): default CacheDir to `tmp/go` to avoid Python collision
+
+- `go/internal/config/config.go`: `CacheDir` default changed from `tmp` to `tmp/go`. Override via `CACHE_DIR` env var still works.
+- Why: both backends used `tmp/<key>_cache.json` with the same keys (`attendance_regular`, `attendance_juniors`, `payments_transactions`, `exceptions_dict`) but different shapes — Python caches post-processed view-model tuples, Go caches raw rows. Whichever wrote last poisoned the cache; running both in parallel produced `ValueError: too many values to unpack (expected 2, got 68)` on Python's `/adults` after the Go server populated `attendance_regular_cache.json` with raw CSV rows.
+- After upgrading: stop the Go server, hit `/flush-cache` on the Python side once (rewrites `tmp/*.json` with correct shapes), then restart `make web-go` — it will use `tmp/go/` going forward. Required for the M5.4 `make parity` workflow which assumes both backends run side-by-side.
+
 ## 2026-05-07 22:37 CEST — feat(py): M5.3 — Python /api/* shadow endpoints
 
 - `app.py`: four new JSON routes (`/api/version`, `/api/adults`, `/api/juniors`, `/api/payments`) mirroring the Go `/api/*` handlers; `_unwrap_view_model_for_api()` helper expands pre-serialised JSON strings and renames `month_labels_json` → `month_labels`, `raw_payments_json` → `raw_payments` to match Go wire contract.