fix: add schema version field to Event type for backward-compatible evolution

[ravimahatonp]

Summary

Closes #1382

Problem

Events defined in event.rs have no version field. Adding or removing fields later risks breaking existing webhook consumers and audit log parsers that expect a fixed shape, with no way to distinguish between schema revisions.

Solution

• Added a version: u32 field to the Event struct with #[serde(default = "default_version")], defaulting to 1.
• New events created via Event::new() are stamped with version: 1.
• Legacy events serialized without a version field deserialize cleanly with the default, ensuring full backward compatibility.
• Added 2 new tests:
  • test_version_in_serialized_json — verifies version appears in JSON output.
  • test_backward_compat_missing_version_defaults_to_1 — verifies legacy JSON without version deserializes to version 1.
• Updated existing tests to assert version == 1.

Why #[serde(default)]?

This ensures that any previously persisted events (e.g., in Redis audit trail) that lack the version key can still be deserialized without errors, defaulting to schema version 1.

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