feature: Add events pagination

[vchurbanov]

Summary

This PR adds cursor-based pagination for events, designed for a dynamic event stream where new events may continuously appear.

The API is intended for practical frontend usage with infinite scrolling and avoids issues typical for offset-based pagination (duplicates, gaps, unstable ordering).

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Pagination approach

Instead of offsets, the API uses a composite cursor based on the last item of the previous page.

Each page response returns:

{
  "limit": 25,
  "has_more": true,
  "next_cursor": "base64(...)"
}

The next_cursor encodes a (timestamp, uuid) pair.
On the next request, the server returns events strictly after this pair, ordered by:

(timestamp, uuid)

This guarantees stable ordering and deterministic pagination, even when timestamps are equal.

The cursor is transferred as Base64 for simplicity and to prevent client-side modification.

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Server-side changes

• Extended /api/events and /api/events/preview with cursor-based pagination
  (backward compatibility preserved)
• Added /api/events/type-counts endpoint to return the current number of events per type
• Implemented ordering by timestamp; since the column is stored as string, casting is used:

  CAST(timestamp AS DECIMAL(20,6))

• Added API documentation and tests for new and updated endpoints
• Removed the previous hard limit of 100 events — events can now be loaded dynamically

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Result

The server now provides a stable and scalable pagination mechanism suitable for infinite scrolling and large, continuously growing event streams.