🐛 fix: normalize AI-generated dates to valid ISO offset datetimes

- add normalizeAiDateString() to coerce bare ISO datetimes, date-only
  strings, and fractional-second variants into offset-aware format
- apply via z.preprocess in AiEventResponseItemSchema so Zod validation
  no longer rejects AI responses missing a timezone offset
- fix system prompt to use toISOString() (unambiguous UTC) and clarify
  expected datetime format for the AI model
- install bun-types and add to tsconfig so bun:test resolves cleanly
- add 8 behaviour-driven tests covering all normalizer edge cases

src/lib/date-normalizer.ts

@@ -0,0 +1,10 @@
+import { formatISO, parseISO } from "date-fns";
+
+const ISO_DATETIME_WITH_OFFSET =
+	/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(Z|[+-]\d{2}:\d{2})$/;
+
+export const normalizeAiDateString = (input: string): string => {
+	if (ISO_DATETIME_WITH_OFFSET.test(input)) return input;
+	const parsed = parseISO(input);
+	return formatISO(parsed);
+};