Replace custom setInterval scheduler with croner

Swap the hand-rolled Scheduler class (setInterval-based) for croner,
a battle-tested cron library with overlap protection and proper cron
expressions. Consolidate lib/jobs/ into a single lib/cron.ts and
remove the unused admin jobs API route.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/api/admin/jobs/[name]/route.ts

@@ -1,25 +0,0 @@
-import type { NextRequest } from "next/server";
-import { NextResponse } from "next/server";
-import { registerJobs } from "@/lib/jobs/registry";
-import { scheduler } from "@/lib/jobs/scheduler";
-
-// Ensure jobs are registered for manual triggering
-registerJobs();
-
-export async function POST(
-  _req: NextRequest,
-  { params }: { params: Promise<{ name: string }> },
-) {
-  const { name } = await params;
-  const jobNames = scheduler.getJobNames();
-
-  if (!jobNames.includes(name)) {
-    return NextResponse.json(
-      { error: `Unknown job: ${name}. Available: ${jobNames.join(", ")}` },
-      { status: 400 },
-    );
-  }
-
-  await scheduler.runNow(name);
-  return NextResponse.json({ ok: true, job: name });
-}