V157: Widen phoenix_kit_annotations_kind_check for the new "image" kind.

PR #660 exposed Etcher 0.9's :image tool in the media viewer's toolbar (media_canvas_viewer.html.heex) without widening the CHECK constraint (or the schema's @kinds) to match — the same regression V130's moduledoc warns about for "marker". Without this, inserting an image annotation is rejected by the DB and silently fails to persist across a reload.

Idempotent: each ADD CONSTRAINT is preceded by DROP CONSTRAINT IF EXISTS on the same prefixed table.

down/1 is conditional, by necessity

Rolling back re-adds the narrower CHECK, and Postgres validates every existing row when a CHECK is added. So the rollback is only possible while no kind = 'image' annotation exists — once a user has drawn one, the old constraint is not a truthful description of the data and there is no correct way for a schema migration to assert it.

down/1 therefore checks first and raises a message naming the row count and the two ways forward, rather than letting the ALTER fail with an opaque 23514 mid-rollback. The alternatives were both worse: deleting or rewriting user annotations is data loss a rollback has no business performing, and NOT VALID would leave a constraint that lies about the rows already in the table.