Mark failed evidence descriptions instead of retrying

Previously the worker reset failed descriptions back to PENDING,
causing infinite retry loops for permanently broken evidence.
Add a FAILED status so these items stop cycling and can be
investigated.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/coredata/migrations/20260326T150000Z.sql

@@ -0,0 +1 @@
+ALTER TYPE evidence_description_status ADD VALUE IF NOT EXISTS 'FAILED';