fix(coprocessor): make previous migration downgrade compatible (#2111)

original migration in commit eb527b7

Author: rudy-6-4

coprocessor/fhevm-engine/db-migration/migrations/20260312174148_downgradable_block_status.sql

@@ -0,0 +1,11 @@
+-- Previous migration
+
+--ALTER TABLE IF EXISTS host_chain_blocks_valid
+--ADD COLUMN IF NOT EXISTS block_status TEXT NOT NULL DEFAULT 'unknown' CHECK (block_status IN ('pending', 'unknown', 'finalized', 'orphaned'));
+--ALTER TABLE IF EXISTS host_chain_blocks_valid
+--ALTER COLUMN block_status DROP DEFAULT;
+
+-- New migration to accept downgrade, default should be dropped at 0.12
+-- Add 'unknown' as default
+ALTER TABLE IF EXISTS host_chain_blocks_valid
+ALTER COLUMN block_status SET DEFAULT 'unknown';