feat(database): add PostgreSQL support with native pgx pool

Add PostgreSQL support to database package alongside existing MySQL support.
Both databases share common infrastructure (pool management, metrics,
transactions) while using database-specific connectors.

database/ changes:
- Add PostgresConfig struct and PostgreSQL connector using pgx/stdlib
- Change MySQL config from DBConfig to *MySQLConfig (pointer)
- Add Config.Validate() to prevent multiple database configs
- Add PostgreSQL connector with secure config building (no password in DSN)
- Add field validation and secure defaults (SSLMode="prefer")
- Support legacy flat PostgreSQL config format for backward compatibility
- Add tests for PostgreSQL configs and validation

New database/pgdb/ package:
- Native pgx connection pool support (*pgxpool.Pool)
- OpenPool() and OpenPoolWithConfigFile() APIs
- CreatePoolConfig() for secure config conversion
- PoolOptions for fine-grained pool control
- Full test coverage and documentation

Security:
- Passwords never exposed in DSN strings
- Set passwords separately in pgx config objects
- Validate all configuration before connection

Architecture:
- Shared code in database/ for both MySQL and PostgreSQL (sql.DB)
- database/pgdb/ for PostgreSQL-specific native pool support

database/pgdb/pool_test.go

@@ -0,0 +1,151 @@
+package pgdb
+
+import (
+	"testing"
+	"time"
+
+	"go.ntppool.org/common/database"
+)
+
+func TestCreatePoolConfig(t *testing.T) {
+	tests := []struct {
+		name    string
+		cfg     *database.PostgresConfig
+		wantErr bool
+	}{
+		{
+			name: "valid config",
+			cfg: &database.PostgresConfig{
+				Host:    "localhost",
+				Port:    5432,
+				User:    "testuser",
+				Pass:    "testpass",
+				Name:    "testdb",
+				SSLMode: "require",
+			},
+			wantErr: false,
+		},
+		{
+			name: "valid config with defaults",
+			cfg: &database.PostgresConfig{
+				Host: "localhost",
+				User: "testuser",
+				Pass: "testpass",
+				Name: "testdb",
+				// Port and SSLMode will use defaults
+			},
+			wantErr: false,
+		},
+		{
+			name: "missing host",
+			cfg: &database.PostgresConfig{
+				User: "testuser",
+				Pass: "testpass",
+				Name: "testdb",
+			},
+			wantErr: true,
+		},
+		{
+			name: "missing user",
+			cfg: &database.PostgresConfig{
+				Host: "localhost",
+				Pass: "testpass",
+				Name: "testdb",
+			},
+			wantErr: true,
+		},
+		{
+			name: "missing database name",
+			cfg: &database.PostgresConfig{
+				Host: "localhost",
+				User: "testuser",
+				Pass: "testpass",
+			},
+			wantErr: true,
+		},
+		{
+			name: "invalid sslmode",
+			cfg: &database.PostgresConfig{
+				Host:    "localhost",
+				User:    "testuser",
+				Pass:    "testpass",
+				Name:    "testdb",
+				SSLMode: "invalid",
+			},
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			poolCfg, err := CreatePoolConfig(tt.cfg)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("CreatePoolConfig() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !tt.wantErr && poolCfg == nil {
+				t.Error("CreatePoolConfig() returned nil config without error")
+			}
+			if !tt.wantErr && poolCfg != nil {
+				// Verify config fields are set correctly
+				if poolCfg.ConnConfig.Host != tt.cfg.Host && tt.cfg.Host != "" {
+					t.Errorf("Expected Host=%s, got %s", tt.cfg.Host, poolCfg.ConnConfig.Host)
+				}
+				if poolCfg.ConnConfig.User != tt.cfg.User {
+					t.Errorf("Expected User=%s, got %s", tt.cfg.User, poolCfg.ConnConfig.User)
+				}
+				if poolCfg.ConnConfig.Password != tt.cfg.Pass {
+					t.Errorf("Expected Password to be set correctly")
+				}
+				if poolCfg.ConnConfig.Database != tt.cfg.Name {
+					t.Errorf("Expected Database=%s, got %s", tt.cfg.Name, poolCfg.ConnConfig.Database)
+				}
+			}
+		})
+	}
+}
+
+func TestDefaultPoolOptions(t *testing.T) {
+	opts := DefaultPoolOptions()
+
+	// Verify expected defaults
+	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.MaxConnLifetime != time.Hour {
+		t.Errorf("Expected MaxConnLifetime=1h, got %v", opts.MaxConnLifetime)
+	}
+	if opts.MaxConnIdleTime != 30*time.Minute {
+		t.Errorf("Expected MaxConnIdleTime=30m, got %v", opts.MaxConnIdleTime)
+	}
+	if opts.HealthCheckPeriod != time.Minute {
+		t.Errorf("Expected HealthCheckPeriod=1m, got %v", opts.HealthCheckPeriod)
+	}
+	if len(opts.ConfigFiles) == 0 {
+		t.Error("Expected ConfigFiles to be non-empty")
+	}
+}
+
+func TestCreatePoolConfigDefaults(t *testing.T) {
+	// Test that defaults are applied correctly
+	cfg := &database.PostgresConfig{
+		Host: "localhost",
+		User: "testuser",
+		Pass: "testpass",
+		Name: "testdb",
+		// Port and SSLMode not set
+	}
+
+	poolCfg, err := CreatePoolConfig(cfg)
+	if err != nil {
+		t.Fatalf("CreatePoolConfig() failed: %v", err)
+	}
+
+	// Verify defaults were applied
+	if poolCfg.ConnConfig.Port != 5432 {
+		t.Errorf("Expected default Port=5432, got %d", poolCfg.ConnConfig.Port)
+	}
+}