refactor: implement native database/sql repositories

[allisson]

Remove github.com/allisson/sqlutil library and replace with native Go database/sql implementations. This change creates separate repository implementations for MySQL and PostgreSQL, providing better control, clarity, and eliminating external dependencies.

Breaking Changes:

• UserRepository split into MySQLUserRepository and PostgreSQLUserRepository
• OutboxEventRepository split into MySQLOutboxEventRepository and PostgreSQLOutboxEventRepository
• Repositories now return interfaces from usecase package instead of concrete types
• DI container updated to instantiate correct repository based on DB_DRIVER config

Changes:

• internal/database/txmanager.go:
  • Remove sqlutil.Querier import
  • Define custom Querier interface matching database/sql
  • Update GetTx() to return new Querier interface
• internal/user/repository/:
  • Delete user_repository.go (unified implementation)
  • Add mysql_user_repository.go with MySQL-specific implementation
    • Uses BINARY(16) for UUID storage with marshal/unmarshal
    • Uses ? placeholders for parameters
    • MySQL-specific unique constraint error detection
  • Add postgresql_user_repository.go with PostgreSQL-specific implementation
    • Uses native UUID type
    • Uses $1, $2, $3... numbered placeholders
    • PostgreSQL-specific unique constraint error detection
  • Add mysql_user_repository_test.go with comprehensive tests
  • Add postgresql_user_repository_test.go with comprehensive tests
  • Delete user_repository_test.go (old tests)
• internal/outbox/repository/:
  • Delete outbox_repository.go (unified implementation)
  • Add mysql_outbox_repository.go with MySQL-specific implementation
  • Add postgresql_outbox_repository.go with PostgreSQL-specific implementation
  • Add mysql_outbox_repository_test.go with comprehensive tests
  • Add postgresql_outbox_repository_test.go with comprehensive tests
  • Delete outbox_repository_test.go (old tests)
• internal/app/di.go:
  • Update Container struct to use interface types for repositories
  • Update UserRepository() to return userUsecase.UserRepository interface
  • Update OutboxRepository() to return userUsecase.OutboxEventRepository interface
  • Update initUserRepository() to switch on DBDriver and instantiate correct implementation
  • Update initOutboxRepository() to switch on DBDriver and instantiate correct implementation
• go.mod:
  • Remove github.com/allisson/sqlutil v1.10.0
  • Remove indirect dependencies: sqlquery, scany, sqlbuilder, xstrings
• Documentation:
  • README.md: Add "Database Repository Pattern" section explaining the architecture
  • README.md: Update all code examples to show database/sql usage
  • README.md: Update project structure to show separate repository files
  • README.md: Update dependencies list removing sqlutil
  • internal/app/README.md: Update dependency graph showing interface pattern
  • internal/app/README.md: Update initialization examples with driver switch logic

Implementation Details:

MySQL Repositories:

• Use BINARY(16) for UUID storage requiring ID.MarshalBinary() for writes and ID.UnmarshalBinary() for reads
• Use ? placeholders for all query parameters
• Detect unique violations by checking for "1062" or "duplicate entry" in error messages
• Use FOR UPDATE SKIP LOCKED syntax for outbox event locking PostgreSQL Repositories:
• Use native UUID type, passing uuid.UUID directly without conversion
• Use numbered placeholders ($1, $2, $3...)
• Detect unique violations by checking for "duplicate key" or "unique constraint"
• Use FOR UPDATE SKIP LOCKED syntax for outbox event locking

Benefits:

• Zero external dependencies for database operations (standard library only)
• Database-specific optimizations (native UUID support in PostgreSQL)
• Explicit SQL queries improve code clarity and maintainability
• Easier to debug and optimize database-specific issues
• Better type safety without abstraction layers
• Reduced dependency tree and faster builds

Testing:

• All existing tests updated and passing
• New tests cover MySQL and PostgreSQL implementations separately
• Tests use sqlmock for database mocking
• Coverage maintained at same level as before

Lint:

• Fixed errcheck warnings by adding //nolint:errcheck to defer rows.Close()
• All golangci-lint checks passing (0 issues)