fix(migration): drop FKs before recreate on MySQL (sc-105349)

CI test-mysql failed with:

  MySQLdb.OperationalError: (1826, "Duplicate foreign key constraint
  name 'fk_dashboard_slices_slice_id_slices'")

Root cause: MySQL scopes foreign-key constraint names per-database,
not per-table (PostgreSQL and SQLite scope per-table). The
``batch_alter_table(... recreate="always", copy_from=...)`` path
used for ``dashboard_slices`` and ``report_schedule_user`` builds
``_alembic_tmp_<table>`` carrying the original FK names from
``copy_from`` while the original table still holds those names — MySQL
rejects the temp-table creation with ERROR 1826.

Fix: on MySQL only, drop the original FK constraints by name before
the ``batch_alter_table`` runs. The ``copy_from`` re-creates them on
the rebuilt table with their original names, so the post-migration
shape is unchanged. On PostgreSQL and SQLite the original code path
still runs unchanged.

Local SQLite tests (44 passed, 1 skipped) still pass; CI will validate
on MySQL.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>