Add DB_MAX conf

[minikill]

Summary

Adds environment variables to configure database connection pool limits, providing better control over database resource usage.

Changes

• Added PBW_DB_MAX_CONNS environment variable (default: 10)
• Added PBW_DB_MAX_IDLE_CONNS environment variable (default: 5)
• Updated connection pool configuration to use these values
• Added documentation in .env.example
• Logs connection pool settings on startup for visibility

Motivation

• Allows users to limit database connections to prevent connection pool exhaustion
• Useful for environments with connection limits (e.g., managed databases)
• Workaround for lib/pq driver issues: The current PostgreSQL driver (lib/pq) has known prepared statement conflicts in connection pools, especially with managed databases like DigitalOcean. Setting up the exact number of connections allows to avoid using connection pools
• Provides flexibility to tune performance vs. stability based on deployment environment

Testing

• ✅ All existing tests pass
• ✅ Linting passes
• ✅ Tested locally with various connection pool configurations
• ✅ Verified single connection mode eliminates prepared statement errors

Issue

#148

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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