fix(activity): compute mock activity timestamps dynamically at access time

[dezzy-music]

Summary

Fixes #707.

Mock activity events computed their timestamps using minutesAgo(n) once when mock-data.ts was initially loaded. On a long-running Next.js production server, the module remains loaded, causing the homepage's 'Live on the platform' feed to display events as hours or days stale.

Changes

• Introduced getRecentActivity(now?: number) to compute timestamps dynamically relative to the current invocation time.
• Provided a dynamic Proxy export for recentActivity to preserve seamless backwards-compatibility for existing call sites while ensuring all accesses receive fresh timestamps.
• Added unit tests in mock-data.test.ts asserting relative timestamp accuracy and dynamic advancing over time.

Verification

• Ran Jest: 8/8 tests pass in mock-data.test.ts.
• Ran ESLint: 0 errors.

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