fix: prevent unauthenticated click fraud on POST /api/links/:id/click

[aarushlohit]

Description
Fixes #139 — Unauthenticated click-count inflation via link ID enumeration. The POST /api/links/:id/click endpoint had no auth or rate limiting, letting attackers enumerate UUIDs and inflate click counters arbitrarily.
Changes Made

• Added clickLimiter — rate limit of 30 requests per 15 minutes per IP using express-rate-limit, consistent with existing limiter patterns in the codebase
• Added IP-based deduplication — in-memory Map keyed by {clientIp}:{linkId} with 1-hour TTL prevents the same visitor from counting multiple clicks on the same link
• Added cleanup on 404 — if the link doesn't exist, the dedup entry is removed so invalid IDs don't poison the dedup map
• Periodic cleanup — setInterval evicts stale dedup entries every 5 minutes to prevent memory leak

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