fix: replace @/ path alias with relative imports in MCP server

The @/ alias resolves via tsconfig but not in production where
Bun runs server files directly. Use relative paths instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/server/mcp/tools/threads.ts

@@ -1,4 +1,4 @@
-import type { db as prodDb } from "@/db/index.ts";
+import type { db as prodDb } from "../../../db/index.ts";
 import {
 	createCandidate,
 	createThread,