feat: add guests search backend + group_size filter

[zaydaanjahangir]

Description

We want to add search functionality and filtering by group size on the guests

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)

Related to #129

What Changed?

• Now we can add search and group_size as filters

Testing & Validation

How this was tested

1. Unit tests
2. Tested locally w/Postman

Notes & Nuances

• ---

• ---

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: ...

[codecov]

Codecov Report

❌ Patch coverage is 23.63636% with 42 lines in your changes missing coverage. Please review.
✅ Project coverage is 11.23%. Comparing base (1740e94) to head (e31a5f4).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
backend/internal/repository/guests.go	0.00%	42 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #218       +/-   ##
===========================================
- Coverage   24.23%   11.23%   -13.00%     
===========================================
  Files          37      103       +66     
  Lines        1267     3347     +2080     
  Branches        0       24       +24     
===========================================
+ Hits          307      376       +69     
- Misses        956     2967     +2011     
  Partials        4        4               

Flag	Coverage Δ
backend	24.52% <23.63%> (+0.29%)	⬆️
mobile	84.00% <ø> (?)
web	0.70% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
backend/internal/handler/guests.go	100.00% <100.00%> (ø)
backend/internal/repository/guests.go	0.00% <0.00%> (ø)

... and 66 files with indirect coverage changes

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[Dao-Ho]

This is HUGE, some comments but overall query looks solid to me!

[Dao-Ho]

LGTM ✅

[zaydaanjahangir]

LGTM ✅

mashallah