feat(api): add input validation to AI endpoints

Add prompt validation to ai-event (non-empty string, max 2000 chars)
and events array length validation to ai-summary (max 100 items)
to prevent abuse and injection attacks.

src/app/api/ai-summary/route.ts

@@ -23,6 +23,12 @@ export async function POST(request: Request) {
 				{ status: 400 },
 			);
 		}
+		if (events.length > 100) {
+			return NextResponse.json(
+				{ error: "Events array must contain 100 or fewer items" },
+				{ status: 400 },
+			);
+		}
 
 		const res = await fetch("https://openrouter.ai/api/v1/chat/completions", {
 			method: "POST",