xorm's Sync drops every index and unique constraint the synced struct
doesn't declare (#3244). Adds a partialSync helper (no drops) and
converts all 65 existing-table struct syncs to it — the four v2.4.0
migrations that wiped users/tasks indexes on every upgrade, plus every
historical one back to 2019, each a latent index wipe and a pgloader
2BP01 abort point for installs upgrading from versions predating it.