Implement declarative base and reusable mixins for database models.

- Add Base class with to_dict() and __repr__() methods
- Add TimestampMixin for automatic created_at/updated_at tracking
- Add SoftDeleteMixin with soft_delete() and restore() methods
- Use UTC timezone for all timestamp fields
- Enable generic model functionality across all tables

Implement async PostgreSQL engine with connection pooling and
lifecycle management.

- Add create_engine() with configurable connection pool
- Add create_test_engine() with NullPool for testing
- Add init_database()/close_database() for app lifecycle
- Add get_engine() singleton accessor
- Configure connection pool with overflow and timeout settings
- Integrate error handling with DatabaseError/InternalError
- Add comprehensive logging for all operations

Implement async session factory with context managers and
FastAPI dependency injection support.

- Add create_session_factory() with configurable options
- Add get_session() context manager with auto-commit/rollback
- Add get_session_dependency() for FastAPI Depends()
- Wrap all errors in DatabaseError for consistent handling
- Enable expire_on_commit=False for post-commit access
- Integrate with engine module for database connection

Implement type-safe generic CRUD repository with comprehensive
query operations.

- Add BaseRepository[ModelType] with Generic[T] for type safety
- Add CRUD methods: get, get_by, get_all, filter with pagination
- Add bulk operations: create_many, update_many, delete_many
- Add helper methods: count, exists, get_or_raise, get_by_or_raise
- Add execute() for custom SQL queries
- Integrate NotFoundError for missing entities
- Add soft delete support via is_deleted filter
- Enable reusable repository pattern across all models

Implement transaction context manager with savepoint support
for nested transactions.

- Add transaction() context manager with auto-commit/rollback
- Add nested transaction support via savepoints
- Wrap all errors in DatabaseError
- Enable manual transaction control in async contexts
- Integrate with session module for database operations

Implement database health checks and connection monitoring.

- Add check_database_health() with connectivity test and latency
- Add get_database_info() with version, connections, pool stats
- Return structured HealthStatus and DatabaseInfo dataclasses
- Measure query execution time for performance monitoring
- Query PostgreSQL system tables for connection statistics
- Enable health endpoint integration for production monitoring

Implement Alembic migration utilities for schema management.

- Add get_alembic_config() for configuration loading
- Add run_migrations() for applying pending migrations
- Add rollback_migration() for reverting changes
- Add create_migration() for generating new migrations
- Add get_migration_history() for tracking applied migrations
- Configure async database URL for Alembic
- Enable programmatic migration control
- Integrate logging for all migration operations

Export database module public interface for clean imports.

- Export engine functions: init_database, close_database, get_engine
- Export session utilities: get_session, get_session_dependency, AsyncSession
- Export base classes: Base, TimestampMixin, SoftDeleteMixin
- Export repository: BaseRepository
- Export transaction: transaction
- Export health: check_database_health, get_database_info
- Export migrations: run_migrations, rollback_migration, create_migration
- Enable clean import syntax: from app.shared.database import ...

Add comprehensive tests for database module structure and imports.

- Add 23 tests covering all database components
- Test utils fallback classes and logger integration
- Test Base model methods (to_dict, __repr__)
- Test TimestampMixin and SoftDeleteMixin attributes
- Test all module imports and public API exports
- Test repository CRUD method existence
- Test migration utility functions
- Verify module structure and interface consistency

Add complete documentation for database module usage and architecture.

- Add Quick Start guide with setup and basic usage
- Document configuration options and environment variables
- Explain Base model and mixin usage patterns
- Document repository pattern with CRUD examples
- Cover session management and dependency injection
- Explain transaction handling and nested transactions
- Document health checks and monitoring
- Cover Alembic migrations and schema management
- Add error handling best practices
- Include testing guidelines and examples
- Add troubleshooting section for common issues