Add messageStatusValidator for stricter message status validation

[larryro]

Summary

The messageValidator uses v.string() for the status field. Consider creating a messageStatusValidator union for stricter validation.

Context

Identified during PR #37 review (CodeRabbit comment #2652223400).

Current State

export const messageValidator = v.object({
  // ...
  status: v.string(),  // Currently permissive
  // ...
});

Proposed Change

Create a union validator with all valid message statuses:

export const messageStatusValidator = v.union(
  v.literal('sent'),
  v.literal('delivered'),
  v.literal('read'),
  v.literal('failed'),
  // ... other statuses
);

Required Work

1. Audit all message status values used across the codebase
2. Identify all sources of message statuses (email providers, internal state, etc.)
3. Create comprehensive messageStatusValidator union
4. Update messageValidator to use the new validator

Additional Notes

Message statuses may come from different sources (email providers, internal state), so need to ensure all possible values are captured before implementing strict validation.