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])

drizzle/migrations/20260511154808_slimy_annihilus/migration.sql

@@ -0,0 +1,13 @@
+ALTER TABLE "files" ADD COLUMN "size" integer DEFAULT 0 NOT NULL;--> statement-breakpoint
+ALTER TABLE "messages" ADD COLUMN "active_child_id" text;--> statement-breakpoint
+UPDATE "messages" m
+SET "active_child_id" = (
+    SELECT child."id"
+    FROM "messages" child
+    WHERE child."parent_message_id" = m."id"
+    ORDER BY child."created_at" ASC
+    OFFSET (m."focused_index" - 1)
+    LIMIT 1
+)
+WHERE m."focused_index" IS NOT NULL AND m."focused_index" > 0;--> statement-breakpoint
+ALTER TABLE "messages" DROP COLUMN "focused_index";