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/engine/executor.rs

@@ -250,6 +250,8 @@ impl FlowExecutor {
                 let new_token_id = Uuid::now_v7();
                 let now = Utc::now();
 
+                let system_user = uuid::Uuid::nil();
+
                 let token_model = token::ActiveModel {
                     id: Set(new_token_id),
                     tenant_id: Set(tenant_id),
@@ -257,6 +259,11 @@ impl FlowExecutor {
                     node_id: Set(node_id.to_string()),
                     status: Set("active".to_string()),
                     created_at: Set(now),
+                    updated_at: Set(now),
+                    created_by: Set(system_user),
+                    updated_by: Set(system_user),
+                    deleted_at: Set(None),
+                    version: Set(1),
                     consumed_at: Set(None),
                 };
                 token_model