feat: wire activity feed to real API data (Bounty #822)

[liqiuniu]

Summary

Wires the Activity Feed component to real API data. Addresses Bounty #822.

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Changes

` (new)

• ` 鈥?fetches activity events from backend
• Type-safe ` interface

` (new)

• React Query hook with 30-second auto-refresh
• Graceful retry policy (retry: 1) for resilience
• Reuses existing TanStack Query patterns

` (modified)

• Imports ` hook
• Passes real activity data to ` component
• Falls back to mock data when API unavailable (existing component behavior)

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Acceptance Criteria

• Activity feed shows real events from the API
• Events update without page refresh (30s polling)
• Shows mock data when API unavailable (graceful fallback)

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Testing

1. Start frontend dev server
2. Navigate to home page
3. Activity feed should display events from ` endpoint
4. Events refresh every 30 seconds automatically

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Related

Bounty #822 鈥?#822