Resolve the current-year sheet tab per fetch instead of freezing it at
startup, so a long-running process follows the New Year rollover without a
restart. Guard against the regression this introduces: a successful fetch
that returns no duty weeks (e.g. a freshly created, empty next-year tab) now
keeps the last-good cache instead of clobbering it with empty data.
Surface a silently-stale cache — which is not a crash, so process-level
supervision can't catch it — by escalating to a loud error log once no sync
has succeeded for 6h (new Store.LastSynced accessor backs the check).
Throttle manual /sync to once per minute across the chat to avoid spamming
the Google Sheets API; the check-and-set is mutex-guarded since Telegram
dispatches each update in its own goroutine.
Update README accordingly.
These were never committed (hidden by status.showUntrackedFiles=no): without
go.mod/go.sum a fresh clone would not build, and the source files committed in
abb38e3 were missing their accompanying tests.
Kamil 'Kamaś' Bruchal