feat: add Convex adapter for server-side event tracking

[bntvllnt]

Context

@vllnt/analytics currently supports client-side tracking with React hooks. Add server-side Convex adapter for backend event tracking.

Scope

New export: @vllnt/analytics/convex

• createConvexTracker(ctx) — track events from Convex mutations/actions
• Stores events in Convex tables (if Convex Component approach) or forwards to PostHog/custom
• Integrates with existing consent management