Achieve 80% Unit Test Coverage

## Overview
Increase unit test coverage to at least 80% for all critical code paths to ensure code quality and reliability.

## Business Value
High test coverage reduces bugs, enables confident refactoring, and documents expected behavior.

## Current State
- Minimal unit tests exist
- Coverage is below 50%
- Critical paths are untested

## Subtasks
- [ ] Audit current test coverage
- [ ] Identify critical paths
- [ ] Write tests for auth layer
- [ ] Write tests for repository layer
- [ ] Write tests for handlers
- [ ] Write tests for services
- [ ] Set up coverage reporting
- [ ] Add coverage to CI

## Implementation Steps
1. Run coverage analysis
2. Identify gaps
3. Write tests for each module
4. Set up coverage reporting
5. Add coverage requirements to CI

## Acceptance Criteria
- [ ] Overall coverage is >= 80%
- [ ] Critical paths have 100% coverage
- [ ] Tests run in CI
- [ ] Coverage reports are generated
- [ ] Tests are fast (<5s total)

## Estimated Effort
- Hours: 40-60
- Complexity: High

No files or tests are named. Start by running a coverage analysis and auditing the auth, repository, handler, and service layers to identify critical-path gaps, then inspect the existing test and CI setup. Done means coverage reaches at least 80% overall, critical paths reach 100%, reports run in CI, and the full test suite stays under five seconds.

索引モデルが issue の本文から書いたものです。