Fix ESLint type import and add better-auth to serverExternalPackages

Prevents Turbopack from statically analyzing better-auth's kysely adapter,
which imports symbols removed in kysely 0.29. Also fixes @typescript-eslint
consistent-type-imports warning on the Db type alias.

https://claude.ai/code/session_014126WHVRT8mftmqkU6dajG

src/server/api/routers/time-entries.ts

@@ -3,7 +3,7 @@ import { eq, and, desc, isNull, isNotNull, gte, lte, or } from "drizzle-orm";
 import { createTRPCRouter, protectedProcedure } from "../trpc";
 import { timeEntries, clients, invoices, invoiceItems } from "~/server/db/schema";
 import { TRPCError } from "@trpc/server";
-import { db } from "~/server/db";
+import type { db } from "~/server/db";
 
 type Db = typeof db;