refactor: replace focusedIndex with activeChildId

- Replace focusedIndex integer with activeChildId string in messages schema
- Add migration to convert existing data and drop focused_index column
- Add  hooks for providers.updatedAt and messageParts.lastUpdatedAt
- Add file size column to files table
- Update message navigation to use child ID-based lookup
- Simplify delete logic by removing focusedIndex math
- Fix file delete endpoint filename ([id[ -> [id])

server/api/file/[id].delete.ts

@@ -0,0 +1,23 @@
+import { and, eq } from "drizzle-orm";
+import { files } from "~~/drizzle/schema";
+import { db } from "~~/server/lib/db";
+
+export default defineEventHandler(async (event) => {
+    await protectRoute(event);
+
+    const userId = event.context.user!.id;
+
+    const id = getRouterParam(event, 'id')!;
+
+    const file = await db.delete(files).where(and(eq(files.id, id), eq(files.userId, userId))).returning();
+
+    if (!file) {
+        throw createError({
+            statusCode: 404,
+            statusMessage: 'Not Found',
+            message: 'File not found',
+        });
+    }
+
+    return { ok: true };
+});