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/schema.ts

@@ -55,7 +55,7 @@ export const providers = pgTable('providers', {
         apiProxyUrl?: string;
     }>().default({}),
     createdAt: timestamp('created_at').notNull().defaultNow(),
-    updatedAt: timestamp('updated_at').notNull().defaultNow(),
+    updatedAt: timestamp('updated_at').notNull().defaultNow().$onUpdate(() => /* @__PURE__ */ new Date()),
 }, (table) => [
     index('providers_userId_idx').on(table.userId),
 ]);
@@ -134,7 +134,7 @@ export const messages = pgTable('messages', {
     generationId: text('generation_id').references(() => generations.id, { onDelete: 'cascade' }),
     role: roleEnum('role').notNull(),
     content: text('content'),
-    focusedIndex: integer('focused_index'),
+    activeChildId: text('active_child_id'),
     deleted: boolean('deleted').default(false),
     updatedAt: timestamp('updated_at').notNull().defaultNow().$onUpdate(() => /* @__PURE__ */ new Date()),
     createdAt: timestamp('created_at').notNull().defaultNow(),
@@ -155,7 +155,7 @@ export const messageParts = pgTable('message_parts', {
     providerOptions: jsonb('provider_options'),
     finished: boolean('finished').notNull().default(false),
     createdAt: timestamp('created_at').notNull().defaultNow(),
-    lastUpdatedAt: timestamp('last_updated_at').notNull().defaultNow(),
+    lastUpdatedAt: timestamp('last_updated_at').notNull().defaultNow().$onUpdate(() => /* @__PURE__ */ new Date()),
 }, (table) => [
     index('message_parts_topicId_idx').on(table.topicId),
     index('message_parts_messageId_idx').on(table.messageId),
@@ -212,6 +212,7 @@ export const files = pgTable('files', {
     userId: text('user_id').notNull(),
     name: text('name').notNull(),
     mimeType: text('mime_type').notNull(),
+    size: integer('size').notNull().default(0),
     url: text('url').notNull(),
     createdAt: timestamp('created_at').notNull().defaultNow(),
 }, (table) => [