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])

utils/message.ts

@@ -22,20 +22,12 @@ export const buildFocusedMessageTree = (messages: Readonly<Message[]>): MessageE
     let focusedMessageTree: MessageEntity[] = [];
 
     for (const message of messages) {
-        if (message.focusedIndex) {
-            if (message.focusedIndex === 0) {
-                focusedMessageTree.push(message);
-                continue;
-            }
-
-            const activeChild = message.children[Math.min(Math.max(message.focusedIndex! - 1, 0), message.children.length - 1)]
+        if (message.activeChildId && message.children.length > 0) {
+            const activeChild = message.children.find(c => c?.id === message.activeChildId);
             if (activeChild) {
                 focusedMessageTree.push(activeChild);
-            } else {
-                focusedMessageTree.push(message);
+                continue;
             }
-
-            continue;
         }
 
         focusedMessageTree.push(message);