feat(activeDid): implement migration to separate active_identity table

- Add migration 003 with data migration logic to prevent data loss
- Create dedicated ActiveIdentity interface in separate file for better architecture
- Implement $getActiveIdentity method in PlatformServiceMixin
- Enhance $updateActiveDid with dual-write pattern for backward compatibility
- Maintain separation of concerns between settings and active identity types
- Follow project architectural pattern with dedicated type definition files

The migration creates active_identity table alongside existing settings,
automatically copying existing activeDid data to prevent user data loss.
Dual-write pattern ensures backward compatibility during transition.

Migration includes:
- Schema creation with proper constraints and indexes
- Automatic data transfer from settings.activeDid to active_identity.activeDid
- Validation to ensure data exists before migration
- Atomic operation: schema and data migration happen together

src/db/tables/activeIdentity.ts

@@ -0,0 +1,14 @@
+/**
+ * ActiveIdentity type describes the active identity selection.
+ * This replaces the activeDid field in the settings table for better
+ * database architecture and data integrity.
+ *
+ * @author Matthew Raymer
+ * @since 2025-08-29
+ */
+
+export interface ActiveIdentity {
+  id: number;
+  activeDid: string;
+  lastUpdated: string;
+}