spike: short link redirect with click counting (Phase 0)

[smmariquit]

Plan: docs/IMPLEMENTATION_PLAN.md §9 Phase 0

Goal

Create a short link, redirect reliably at the edge, and persist accurate click counts.

Tasks

• packages/link-core: code generation, uniqueness, expiry optional
• Edge or API redirect GET /l/{code} → 302
• Click log (Postgres increment or Redis buffer → aggregate)
• Minimal dashboard: create link, view count

Definition of done

• 100 sequential redirects report correct count
• p95 redirect latency measured on staging

Out of scope

• Events, registration, auth