feat: add platform stats, stream leaderboard and archiving - #901

Summary

This PR adds platform analytics endpoints and sender-controlled stream archiving.

Platform stats

• Adds GET /api/stats.

• Returns:

  • total_streams
  • active_streams
  • completed_streams
  • canceled_streams
  • total_vested_by_asset for USDC and XLM
  • unique_senders
  • unique_recipients

• Stats reflect the current database state.

• Adds 60-second caching.

• Keeps the endpoint public without authentication.

• Updates the frontend stats widget to consume /api/stats.

Stream leaderboard

• Adds GET /api/leaderboard.

• Supports:

  • top_senders
  • top_recipients
  • largest_streams

• Top senders are ranked by total amount streamed.

• Top recipients are ranked by total amount received.

• Largest streams are ranked by totalAmount.

• Supports a configurable limit, defaulting to 10 and validated between 1 and 50.

• Adds 5-minute caching.

• Adds X-Cache response headers for cache HIT/MISS visibility.

Stream archiving

• Allows a stream sender to archive completed or canceled streams.
• Archived streams are excluded from default stream listings.
• GET /api/streams?include_archived=true continues to return archived streams.
• Archived streams remain accessible through direct stream ID lookup.
• Archive state is preserved during subsequent stream updates/synchronization.
• Sender and recipient default listings exclude archived streams.
• Relevant stream list caches are invalidated when a stream is archived.

Testing

• git diff --cached --check passes.
• Backend TypeScript build is currently blocked by pre-existing syntax errors in the existing Soroban createStream() implementation in backend/src/services/streamStore.ts.
• Frontend dependency installation/build is currently blocked by pre-existing frontend package/lockfile issues.
• These unrelated repository issues were intentionally left unchanged to keep this PR scoped to the assigned features.

Closes #704
Closes #707
Closes #715