Add GET /stats/top (most-clicked links leaderboard)

Right now the only way to see how a link is performing is GET /:code/stats, one code at a time -- you have to already know which short code you're asking about. There's no way to answer "which of our links are actually getting clicked?" without querying the database by hand. That's a real gap for anyone using this service to track campaign links, social posts, or anything where "what's working?" is the whole point of tracking clicks in the first place -- right now that question requires direct DB access, not the API.

GET /stats/top is registered as a stub in src/routes/redirect.js (right before GET /:code). It compiles and the route exists, but its handler unconditionally throws -- hitting it always returns a 500, regardless of query parameters.

npm run dev curl -s "http://localhost:3003/stats/top?limit=2"

-> 500 Internal Server Error

This is a straightforward read: one Prisma query with orderBy: { clicks: 'desc' } and a take: limit. The part actually worth thinking through is the limit query parameter -- it arrives as a string (or is absent entirely), and it's client-controlled, so it needs to be parsed and clamped defensively (an unbounded or non-numeric limit should never reach Prisma's take unvalidated).

Look at how GET /:code/stats (right below where you'll add this) already shapes a Prisma response into a small JSON object -- you want the same idea, but for many rows instead of one, using prisma.link.findMany with an orderBy and a take instead of findUnique. Parse req.query.limit with Number(...), and think about what should happen if it's missing, non-numeric, zero, negative, or absurdly large.