Remove `priority` column from runs table

User story

As a developer, I want to remove the deprecated priority column from the
runs table now that queue-based claiming has replaced it, so the codebase
doesn't carry dead code.

Details

The priority column (Ecto.Enum with values [immediate: 0, normal: 1])
was the original mechanism for run ordering. It has been superseded by the
queue column and queue-based claim preferences.

This is a cleanup issue to be done in a release after the fast lanes
feature (Issues 1-5) is deployed and stable.

What to remove:

• Migration: ALTER TABLE runs DROP COLUMN priority
• field :priority from Run schema
• Priority references in Run.for/2, Run.new/1, Run.changeset/2
• Priority ordering in the Query module (any ORDER BY r.priority clauses)
• Any remaining priority-related test fixtures or assertions

What to keep:

• The queues || ["manual", "*"] fallback in WorkerChannel can stay for
  safety (handles old workers that haven't been updated)

Implementation notes

• Search the codebase for all references to :priority and :immediate on
  the Run schema to ensure nothing is missed
• The migration should be straightforward — priority has a default value so
  no data migration is needed, just drop the column
• Any RunQueue implementations (including third-party or extension-based ones)
  that reference priority ordering should be updated

Release notes

Removed the deprecated priority column from the runs table. Run ordering is
now fully handled by the queue-based claiming system introduced in the fast
lanes feature.

User acceptance criteria

• priority column no longer exists on the runs table
• No references to priority or :immediate remain in run-related code
• All existing tests pass (with priority references removed/updated)
• Workers continue to function normally after the migration