ALTER TABLE calls inside _migrate_add_optional_columns were guarded by a
snapshot of PRAGMA table_info taken at function entry. When the gateway
dispatcher opens the kanban DB twice per tick (once in _tick_once_for_board
and once via init_db's discard-and-reconnect path), a second connection can
run the same migration before the first one commits, causing:
sqlite3.OperationalError: duplicate column name: consecutive_failures
This crashed the dispatcher on every first tick after a gateway restart
(subsequent ticks succeeded because the columns were then present).
Fix: introduce _add_column_if_missing() which wraps ALTER TABLE in a
try/except that swallows OperationalError whose message contains
'duplicate column name'. All ALTER TABLE calls in
_migrate_add_optional_columns are routed through this helper.
Closes#21708
78698381af