Use separate migrations for creating and referencing new enum value, to work around known issue

Author: rhigman

thoth-api/migrations/20260414_v1.0.0/down.sql

@@ -0,0 +1,2 @@
+BEGIN;
+COMMIT;

thoth-api/migrations/20260414_v1.0.0/up.sql

@@ -0,0 +1 @@
+ALTER TYPE file_type ADD VALUE IF NOT EXISTS 'accessibility_report';

thoth-api/migrations/20260415_v1.0.0/metadata.toml

@@ -1,5 +1,3 @@
-ALTER TYPE file_type ADD VALUE IF NOT EXISTS 'accessibility_report';
-
 ALTER TABLE file DROP CONSTRAINT IF EXISTS file_type_check;
 ALTER TABLE file_upload DROP CONSTRAINT IF EXISTS file_upload_type_check;