Add consistent API request logging and a themed 404 page, bump to 0.4.0

Every app/api route handler now goes through withRouteLogging, which logs
a correlated reqId/method/path/status/duration for each request and turns
any uncaught error into a logged stack trace plus a clean JSON 500 instead
of Next's default opaque failure. proxy.ts logs rejected auth attempts, and
instrumentation.ts's onRequestError catches anything that still escapes a
route handler. Also adds a minimal not-found page matching the app's card
styling.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/api/with-route-logging.ts

@@ -0,0 +1,43 @@
+import { NextResponse } from "next/server";
+import { randomUUID } from "node:crypto";
+import { createLogger } from "@/lib/logger";
+
+const log = createLogger("api");
+
+type RouteHandler<Ctx> = (req: Request, ctx: Ctx) => Promise<Response> | Response;
+
+/**
+ * Wraps an App Router route handler with consistent request/response logging:
+ * a reqId (also echoed as `x-request-id`), method/path, status, and duration on
+ * every call, plus a logged stack trace and JSON 500 for anything the handler
+ * doesn't catch itself. Every export in app/api/** goes through this so log
+ * shape and error handling stay uniform across routes instead of being
+ * reimplemented per file.
+ */
+export function withRouteLogging<Ctx = unknown>(routeName: string, handler: RouteHandler<Ctx>): RouteHandler<Ctx> {
+  return async (req, ctx) => {
+    const reqId = randomUUID();
+    const start = performance.now();
+    const { pathname, search } = new URL(req.url);
+    const reqLog = log.child({ reqId, route: routeName, method: req.method });
+
+    reqLog.debug("request start", { path: pathname + search });
+
+    try {
+      const res = await handler(req, ctx);
+      const durationMs = Math.round(performance.now() - start);
+      const level = res.status >= 500 ? "error" : res.status >= 400 ? "warn" : "info";
+      reqLog[level]("request end", { status: res.status, durationMs });
+      res.headers.set("x-request-id", reqId);
+      return res;
+    } catch (err) {
+      const durationMs = Math.round(performance.now() - start);
+      reqLog.error("request failed", {
+        durationMs,
+        error: err instanceof Error ? err.message : String(err),
+        stack: err instanceof Error ? err.stack : undefined,
+      });
+      return NextResponse.json({ error: "internal error", reqId }, { status: 500 });
+    }
+  };
+}