feat: migrate all ID fields to UUIDv7

[allisson]

Replace auto-incrementing integer primary keys with UUIDv7 across the entire application for improved scalability, global uniqueness, and time-ordered sorting capabilities.

Changes:

Database Migrations:

• PostgreSQL: Change SERIAL to UUID type for all primary keys
• MySQL: Change BIGINT AUTO_INCREMENT to BINARY(16) for all primary keys
• Update both users and outbox_events table migrations

Domain Models:

• user.User: Change ID field from int64 to uuid.UUID
• outbox.OutboxEvent: Change ID field from int64 to uuid.UUID
• user/http/dto.UserResponse: Update ID field to uuid.UUID

Application Code:

• Generate UUIDs in application using uuid.NewV7() instead of database auto-increment
• User creation: Generate UUID before database insertion
• OutboxEvent creation: Generate UUID before database insertion
• Update all repository method signatures to accept uuid.UUID

Repository Layer:

• user/repository: GetByID() now accepts uuid.UUID parameter
• All repository implementations compatible with UUID storage

Use Case Layer:

• user/usecase: Update interfaces and implementations to use uuid.UUID
• GetUserByID() signature changed to accept uuid.UUID

Tests:

• Update all test files to use uuid.Must(uuid.NewV7())
• Update mock implementations for UUID compatibility
• worker/event_worker_test: Use UUIDs in test fixtures
• user/repository_test: Use UUIDs in test cases
• user/usecase_test: Update mock methods and assertions
• http/http_test: Update handler tests for UUID responses

Dependencies:

• Add github.com/google/uuid v1.6.0

Documentation:

• Add comprehensive UUIDv7 section to README.md
• Document benefits: time-ordering, global uniqueness, scalability
• Provide implementation examples for new domains
• Update code examples throughout README

Benefits:

• Time-ordered IDs maintain temporal sorting
• Globally unique across distributed systems
• Better database index performance than random UUIDs
• No centralized ID generation required
• Merge-friendly for database consolidation scenarios

Breaking Changes:

• All ID fields changed from int64 to uuid.UUID
• API responses now return UUIDs as strings
• Database schema requires migration