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-sql/migration.ts

@@ -124,6 +124,33 @@ const MIGRATIONS = [
       ALTER TABLE contacts ADD COLUMN iViewContent BOOLEAN DEFAULT TRUE;
     `,
   },
+  {
+    name: "003_active_did_separate_table",
+    sql: `
+      -- Create new active_identity table with proper constraints
+      CREATE TABLE IF NOT EXISTS active_identity (
+        id INTEGER PRIMARY KEY CHECK (id = 1),
+        activeDid TEXT NOT NULL,
+        lastUpdated TEXT NOT NULL DEFAULT (datetime('now')),
+        FOREIGN KEY (activeDid) REFERENCES accounts(did) ON DELETE CASCADE
+      );
+
+      -- Add performance indexes
+      CREATE INDEX IF NOT EXISTS idx_active_identity_activeDid ON active_identity(activeDid);
+      CREATE UNIQUE INDEX IF NOT EXISTS idx_active_identity_single_record ON active_identity(id);
+
+      -- Insert default record (will be updated during migration)
+      INSERT OR IGNORE INTO active_identity (id, activeDid, lastUpdated) VALUES (1, '', datetime('now'));
+
+      -- MIGRATE EXISTING DATA: Copy activeDid from settings to active_identity
+      -- This prevents data loss when migration runs on existing databases
+      UPDATE active_identity 
+      SET activeDid = (SELECT activeDid FROM settings WHERE id = 1), 
+          lastUpdated = datetime('now')
+      WHERE id = 1 
+      AND EXISTS (SELECT 1 FROM settings WHERE id = 1 AND activeDid IS NOT NULL AND activeDid != '');
+    `,
+  },
 ];
 
 /**