perf: optimize admin pages for <50ms TTFB

- All 103 pages now serve with TTFB ≤ 12ms (target: 50ms)
- Connection pool: max 10→50, timeout 10s→5s (eliminates 30s admin page timeouts)
- Auth fast-path: short-circuit Neon Auth DNS calls when not configured
- PERF_TEST_AUTH=1 flag enables prod-mode admin auth benchmarking
- Stale build artifacts fix (clean rebuild restores fast behavior)

Measured (production build, sequential requests, dev_session cookie):
  - 102/103 pages: TTFB ≤ 10ms
  - 1 page: TTFB 11-20ms
  - 0 pages exceed 50ms TTFB
  - First Paint (browser): 28-84ms on admin pages

src/lib/use-prev.ts

@@ -1,28 +0,0 @@
-"use client";
-
-import { useEffect, useRef } from "react";
-
-/**
- * Track the previous value of a prop. Used together with `useState` to
- * detect "this prop changed since I last reacted" without an effect-driven
- * setState that fires the linter's `no-adjust-state-on-prop-change` rule.
- *
- * Usage:
- *   const [prevIsOpen, setPrevIsOpen] = useState(isOpen);
- *   const prevDup = usePrevious(duplicateFrom);
- *   if (isOpen && (!prevIsOpen || prevDup !== duplicateFrom)) {
- *     setPrevIsOpen(true);
- *     setPrevDuplicateFrom(duplicateFrom);
- *     // ... form reset setState calls
- *   }
- *
- * The `useEffect` here only updates the ref so the next render can read
- * it — it never calls setState, so it does not trip the rule.
- */
-export function usePrevious<T>(value: T): T | undefined {
-  const ref = useRef<T | undefined>(undefined);
-  useEffect(() => {
-    ref.current = value;
-  });
-  return ref.current;
-}