fix(security): resolve audit findings and compilation errors (Phase 6)

Security fixes:
- Add startup warning for default JWT secret in config
- Add enum validation for priority, recipient_type, channel fields
- Add pagination size cap (max 100) via safe_page_size()
- Return generic "权限不足" instead of specific permission names

Compilation fixes:
- Fix missing standard fields in ActiveModel for tokens/process_variables
- Fix migration imports for Statement/DatabaseBackend/Uuid
- Add version_field to process_definition ActiveModel

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/erp-workflow/src/service/definition_service.rs

@@ -93,6 +93,7 @@ impl DefinitionService {
             created_by: Set(operator_id),
             updated_by: Set(operator_id),
             deleted_at: Set(None),
+            version_field: Set(1),
         };
         model
             .insert(db)