This PR migrates `noteTarget` and `taskTarget` to morph relations behind
separate feature flags, following the Attachment/TimelineActivity
pattern.
It introduces the `IS_NOTE_TARGET_MIGRATED` and
`IS_TASK_TARGET_MIGRATED` flags, updates standard field metadata and
indexes to use morph relations, and adds two **1.17 workspace
migrations** that:
- rename `noteTarget.*Id` / `taskTarget.*Id` columns to `target*Id`
- convert the corresponding field metadata to `MORPH_RELATION` with a
shared `morphId`
On the frontend, note/task target read and write paths switch to
`target*Id` when the respective flag is enabled. Deleted targets are
filtered on reload to prevent reappearing relations.
