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/config_test.go

@@ -56,9 +56,9 @@ func TestMonitorConfigOptions(t *testing.T) {
 }
 
 func TestConfigStructures(t *testing.T) {
-	// Test that configuration structures can be created and populated
+	// Test that MySQL configuration structures can be created and populated
 	config := Config{
-		MySQL: DBConfig{
+		MySQL: &MySQLConfig{
 			DSN:    "user:pass@tcp(localhost:3306)/dbname",
 			User:   "testuser",
 			Pass:   "testpass",
@@ -79,3 +79,118 @@ func TestConfigStructures(t *testing.T) {
 		t.Errorf("Expected DBName='testdb', got '%s'", config.MySQL.DBName)
 	}
 }
+
+func TestPostgresConfigStructures(t *testing.T) {
+	// Test that PostgreSQL configuration structures can be created and populated
+	config := Config{
+		Postgres: &PostgresConfig{
+			Host:    "localhost",
+			Port:    5432,
+			User:    "testuser",
+			Pass:    "testpass",
+			Name:    "testdb",
+			SSLMode: "require",
+		},
+	}
+
+	if config.Postgres.Host != "localhost" {
+		t.Errorf("Expected Host='localhost', got '%s'", config.Postgres.Host)
+	}
+	if config.Postgres.Port != 5432 {
+		t.Errorf("Expected Port=5432, got %d", config.Postgres.Port)
+	}
+	if config.Postgres.User != "testuser" {
+		t.Errorf("Expected User='testuser', got '%s'", config.Postgres.User)
+	}
+	if config.Postgres.Pass != "testpass" {
+		t.Errorf("Expected Pass='testpass', got '%s'", config.Postgres.Pass)
+	}
+	if config.Postgres.Name != "testdb" {
+		t.Errorf("Expected Name='testdb', got '%s'", config.Postgres.Name)
+	}
+	if config.Postgres.SSLMode != "require" {
+		t.Errorf("Expected SSLMode='require', got '%s'", config.Postgres.SSLMode)
+	}
+}
+
+func TestLegacyPostgresConfig(t *testing.T) {
+	// Test that legacy flat PostgreSQL format can be created
+	config := Config{
+		User:    "testuser",
+		Pass:    "testpass",
+		Host:    "localhost",
+		Port:    5432,
+		Name:    "testdb",
+		SSLMode: "require",
+	}
+
+	if config.User != "testuser" {
+		t.Errorf("Expected User='testuser', got '%s'", config.User)
+	}
+	if config.Name != "testdb" {
+		t.Errorf("Expected Name='testdb', got '%s'", config.Name)
+	}
+}
+
+func TestConfigValidation(t *testing.T) {
+	tests := []struct {
+		name    string
+		config  Config
+		wantErr bool
+	}{
+		{
+			name: "valid mysql config",
+			config: Config{
+				MySQL: &MySQLConfig{DSN: "test"},
+			},
+			wantErr: false,
+		},
+		{
+			name: "valid postgres config",
+			config: Config{
+				Postgres: &PostgresConfig{User: "test", Host: "localhost", Name: "test"},
+			},
+			wantErr: false,
+		},
+		{
+			name: "valid legacy postgres config",
+			config: Config{
+				User: "test",
+				Host: "localhost",
+				Name: "testdb",
+			},
+			wantErr: false,
+		},
+		{
+			name: "both mysql and postgres set",
+			config: Config{
+				MySQL:    &MySQLConfig{DSN: "test"},
+				Postgres: &PostgresConfig{User: "test"},
+			},
+			wantErr: true,
+		},
+		{
+			name: "mysql and legacy postgres set",
+			config: Config{
+				MySQL: &MySQLConfig{DSN: "test"},
+				User:  "test",
+				Name:  "testdb",
+			},
+			wantErr: true,
+		},
+		{
+			name:    "no config set",
+			config:  Config{},
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := tt.config.Validate()
+			if (err != nil) != tt.wantErr {
+				t.Errorf("Validate() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}