#110 Store original Python DSL definition alongside JSON workflow definition

Problem: When workflows are submitted via API, only the JSON representation is stored in workflow_definition table. The original Python DSL source code is lost. Requirements: - Non-breaking change (existing workflows continue to work) - Scalable storage (DSL can be large) - Optional field (not all submissions have DSL source) - Linkable to workflow_definition Approaches to evaluate: 1. Add dsl_source TEXT column to workflow_definition 2. Use artifact system with dsl_artifact_id FK 3. Store in metadata JSON field 4. Separate workflow_dsl_sources table