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/logger.ts

@@ -34,14 +34,21 @@ export interface Logger {
   info(message: string, fields?: LogFields): void;
   warn(message: string, fields?: LogFields): void;
   error(message: string, fields?: LogFields): void;
+  /** Returns a logger that merges `bindings` into every call's fields - for attaching
+   * per-request context (reqId, route) without threading it through every log call. */
+  child(bindings: LogFields): Logger;
 }
 
 /** Scoped logger. `scope` is the bracketed tag, e.g. createLogger("api") -> "[api]". */
-export function createLogger(scope: string): Logger {
+export function createLogger(scope: string, bindings?: LogFields): Logger {
+  const log = (level: LogLevel, message: string, fields?: LogFields) =>
+    write(level, scope, message, bindings || fields ? { ...bindings, ...fields } : undefined);
+
   return {
-    debug: (message, fields) => write("debug", scope, message, fields),
-    info: (message, fields) => write("info", scope, message, fields),
-    warn: (message, fields) => write("warn", scope, message, fields),
-    error: (message, fields) => write("error", scope, message, fields),
+    debug: (message, fields) => log("debug", message, fields),
+    info: (message, fields) => log("info", message, fields),
+    warn: (message, fields) => log("warn", message, fields),
+    error: (message, fields) => log("error", message, fields),
+    child: (extra) => createLogger(scope, { ...bindings, ...extra }),
   };
 }