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/pool_test.go

@@ -115,8 +115,8 @@ func TestDefaultPoolOptions(t *testing.T) {
 	if opts.MinConns != 0 {
 		t.Errorf("Expected MinConns=0, got %d", opts.MinConns)
 	}
-	if opts.MaxConns != 25 {
-		t.Errorf("Expected MaxConns=25, got %d", opts.MaxConns)
+	if opts.MaxConns != 4 {
+		t.Errorf("Expected MaxConns=4 (pgxpool default), got %d", opts.MaxConns)
 	}
 	if opts.MaxConnLifetime != time.Hour {
 		t.Errorf("Expected MaxConnLifetime=1h, got %v", opts.MaxConnLifetime)