Add KB fail state migration: fail_count and last_failed_at columns

New migration 6.3.0-01-kb-fail-state to add columns to kb_documents
for intelligent backoff retry logic.

migrations/6.3.0-01-kb-fail-state/down.sql

@@ -0,0 +1,8 @@
+-- ============================================
+-- Rollback KB Fail State
+-- ============================================
+
+DROP INDEX IF EXISTS idx_kb_documents_fail;
+ALTER TABLE kb_documents 
+    DROP COLUMN IF EXISTS fail_count,
+    DROP COLUMN IF EXISTS last_failed_at;

migrations/6.3.0-01-kb-fail-state/up.sql

@@ -0,0 +1,14 @@
+-- ============================================
+-- KB Documents Fail State
+-- Version: 6.3.0
+-- ============================================
+-- Add fail_count and last_failed_at to kb_documents
+-- for intelligent backoff retry logic
+
+ALTER TABLE kb_documents 
+    ADD COLUMN IF NOT EXISTS fail_count INT NOT NULL DEFAULT 0,
+    ADD COLUMN IF NOT EXISTS last_failed_at TIMESTAMPTZ;
+
+CREATE INDEX IF NOT EXISTS idx_kb_documents_fail 
+    ON kb_documents(bot_id, collection_name, fail_count) 
+    WHERE fail_count > 0;