fix(kanban): make legacy task migration idempotent

[ht1072]

Summary

• refresh the task-column snapshot after early additive migrations
• make the legacy spawn_failures / last_spawn_error migration path idempotent
• avoid re-adding later columns when opening partially migrated databases

Why

Some existing DBs are only partially migrated. _migrate_add_optional_columns() snapshots the schema once at entry, then performs multiple ALTER TABLE steps. On partially migrated DBs that already contain later columns, using the stale initial snapshot can make the legacy-column migration path try to add columns that already exist. Re-reading the schema after the early additive migrations makes the function safe to run repeatedly.

Test Plan

• local migration smoke verification against the patched code path

[ht1072]

Keeping this PR open as a small idempotency fix for partially migrated legacy kanban task DBs. If there's an existing migration thread this should fold into instead, I'm happy to collapse it rather than keep a duplicate patch alive.

[teknium1]

Merged via PR #28397 (cherry-picked onto current main with your authorship preserved via rebase-merge — commit 0b547ae). Thanks for the contribution!