[FEATURE]: Add Configurable Database Connection Pool Limits

[minikill]

Problem

The application currently hardcodes database connection pool settings (SetMaxOpenConns(10)), which can cause issues in certain environments:

1. Managed databases with connection limits (e.g., DigitalOcean) may have strict connection quotas
2. lib/pq driver prepared statement conflicts: With connection pooling, users experience "pq: unnamed prepared statement does not exist" errors, especially on managed PostgreSQL instances (unnamed prepared statement does not exist ERROR on production/staging servers lib/pq#889)
3. No way to tune performance based on deployment environment

Ideal Solution (Long-term)

Switch to a more modern PostgreSQL driver that properly handles prepared statements with connection pooling, such as:

• pgx - Actively maintained with better connection pooling support
• pgxpool - Built specifically for connection pooling scenarios

Proposed Short-term Workaround

Add environment variables to make connection pool settings configurable:

• PBW_DB_MAX_CONNS (default: 10)
• PBW_DB_MAX_IDLE_CONNS (default: 5)

This allows users to:

• Limit connections to match their database plan
• Set to 1 connection to work around lib/pq prepared statement issues
• Tune for their specific workload