fix(pgdb): align pool defaults with pgxpool

Change MaxConns default from 25 to 4 to match pgxpool defaults.
This ensures consistent behavior between DATABASE_URI and config
file modes, and removes the warning log when using URI mode.

Defaults now reference: https://pkg.go.dev/github.com/jackc/pgx/v5/pgxpool#Config

database/pgdb/CLAUDE.md

@@ -75,13 +75,17 @@ sslmode: prefer
 
 ### PoolOptions
 
+Defaults match [pgxpool defaults](https://pkg.go.dev/github.com/jackc/pgx/v5/pgxpool#Config):
+
 - `ConfigFiles` - List of config file paths to search (default: `database.yaml`, `/vault/secrets/database.yaml`)
 - `MinConns` - Minimum connections (default: 0)
-- `MaxConns` - Maximum connections (default: 25)
+- `MaxConns` - Maximum connections (default: 4)
 - `MaxConnLifetime` - Connection lifetime (default: 1 hour)
 - `MaxConnIdleTime` - Idle timeout (default: 30 minutes)
 - `HealthCheckPeriod` - Health check interval (default: 1 minute)
 
+For higher connection limits, set via `PoolOptions` or URI query parameter `?pool_max_conns=25`.
+
 ### PostgreSQL Config Fields
 
 - `host` - Database host (required)
@@ -108,7 +112,7 @@ Pool settings can be included in the URI query string:
 - `pool_max_conn_lifetime`, `pool_max_conn_idle_time`
 - `pool_health_check_period`
 
-When using `DATABASE_URI`, `PoolOptions` are ignored and all settings come from the URI.
+When using `DATABASE_URI`, pool settings come from the URI. Since `PoolOptions` defaults match pgxpool defaults, behavior is consistent whether using URI or config files.
 
 Example with CloudNativePG:
 ```yaml