feat/database-integration

[borisdvlpr]

In this PR, the following changes were made:

• Introduce new database schema migrations for environments, flags, variants, targeting rules and audit log tables
• Add database migration support using golang-migrate and integrated it into the application startup flow
• Implement a helper to run Postgres schema migrations programmatically
• Create a db package with a NewConnectionPool function for managing database connections
• Add configuration support for database connection properties
• Replace lib/pq with pgx/stdlib to reduce dependencies and improve consistency with the connection pool
• Fix middleware order to correctly capture request IDs in logs
• Add a readiness endpoint in the health handler along with required setup
• Minor improvements for consistency and performance