fix: migration 020 - make template column nullable

Issue: template has NOT NULL constraint but pipeline-type prompts
don't use template (they use stages JSONB instead).

Solution: ALTER COLUMN template DROP NOT NULL before inserting
pipeline configs into ai_prompts.

backend/migrations/020_unified_prompt_system.sql

@@ -2,13 +2,17 @@
 -- Consolidate ai_prompts and pipeline_configs into single system
 -- Type: 'base' (reusable building blocks) or 'pipeline' (workflows)
 
--- Step 1: Add new columns to ai_prompts
+-- Step 1: Add new columns to ai_prompts and make template nullable
 ALTER TABLE ai_prompts
 ADD COLUMN IF NOT EXISTS type VARCHAR(20) DEFAULT 'pipeline',
 ADD COLUMN IF NOT EXISTS stages JSONB,
 ADD COLUMN IF NOT EXISTS output_format VARCHAR(10) DEFAULT 'text',
 ADD COLUMN IF NOT EXISTS output_schema JSONB;
 
+-- Make template nullable (pipeline-type prompts use stages instead)
+ALTER TABLE ai_prompts
+ALTER COLUMN template DROP NOT NULL;
+
 -- Step 2: Migrate existing single-prompts to 1-stage pipeline format
 -- All existing prompts become single-stage pipelines with inline source
 UPDATE ai_prompts