feat: Add POST guests endpoint

[zaydaanjahangir]

Description

Added POST endpoint for guests as a part of the whole guest entity integration (#68)

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (code improvement without changing functionality)
• Documentation update
• Configuration/infrastructure change
• Performance improvement
• Test coverage improvement

Related Issue(s)

Addresses part of (#68)

What Changed?

• Adds POST /api/v1/guests endpoint for guest creation
• Implements end-to-end create flow with validation, persistence, and defaults
• Includes unit tests for the guest creation handler

Testing & Validation

How this was tested

1. Tested insertions into local Supabase
2. Unit Tests

Screenshots/Recordings

Unfinished Work & Known Issues

• None, this PR is complete and production-ready
• The following items are intentionally deferred:
  • GET guests
  • PUT guests

Notes & Nuances

• Fixed the failing test cases for timezone (including the user handler test for timezone)

Pre-Merge Checklist

Code Quality

• Code follows the project's style guidelines and conventions
• Self-review completed (I've reviewed my own code for obvious issues)
• No debugging code, console logs, or commented-out code left behind
• No merge conflicts with the base branch
• Meaningful commit messages that explain the "why"

Testing & CI

• All CI checks are passing
• All new and existing tests pass locally
• Test coverage hasn't decreased (or decrease is justified)
• Linting passes without errors

Documentation

• Code is self-documenting or includes helpful comments for complex logic
• API documentation updated (if backend endpoints changed)
• Type definitions are accurate and up-to-date

Reviewer Notes

• Areas needing extra attention: ...
• Questions for reviewers: ...

[Dao-Ho]

one smol cmt to throw a specific conflict error on inserts, otherwise LGTM 🔥

[Dao-Ho]

ah, I should've clarifiedm we have a errs/repository.go for defining already exists constants.
You can specify the err message in the handler and throw a http error once you catch this repository error. Lmk if this make sense

[Dao-Ho]

one more 🙏