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.go

@@ -1,6 +1,7 @@
 package database
 
 import (
+	"fmt"
 	"os"
 	"time"
 
@@ -9,15 +10,67 @@ import (
 
 // Config represents the database configuration structure
 type Config struct {
-	MySQL DBConfig `yaml:"mysql"`
+	// MySQL configuration (use this OR Postgres, not both)
+	MySQL *MySQLConfig `yaml:"mysql,omitempty"`
+	// Postgres configuration (use this OR MySQL, not both)
+	Postgres *PostgresConfig `yaml:"postgres,omitempty"`
+
+	// Legacy flat PostgreSQL format (deprecated, for backward compatibility only)
+	// If neither MySQL nor Postgres is set, these fields will be used for PostgreSQL
+	User    string `yaml:"user,omitempty"`
+	Pass    string `yaml:"pass,omitempty"`
+	Host    string `yaml:"host,omitempty"`
+	Port    uint16 `yaml:"port,omitempty"`
+	Name    string `yaml:"name,omitempty"`
+	SSLMode string `yaml:"sslmode,omitempty"`
 }
 
-// DBConfig represents the MySQL database configuration
-type DBConfig struct {
-	DSN    string `default:"" flag:"dsn" usage:"Database DSN"`
-	User   string `default:"" flag:"user"`
-	Pass   string `default:"" flag:"pass"`
-	DBName string // Optional database name override
+// MySQLConfig represents the MySQL database configuration
+type MySQLConfig struct {
+	DSN    string `yaml:"dsn" default:"" flag:"dsn" usage:"Database DSN"`
+	User   string `yaml:"user" default:"" flag:"user"`
+	Pass   string `yaml:"pass" default:"" flag:"pass"`
+	DBName string `yaml:"name,omitempty"` // Optional database name override
+}
+
+// PostgresConfig represents the PostgreSQL database configuration
+type PostgresConfig struct {
+	User    string `yaml:"user"`
+	Pass    string `yaml:"pass"`
+	Host    string `yaml:"host"`
+	Port    uint16 `yaml:"port"`
+	Name    string `yaml:"name"`
+	SSLMode string `yaml:"sslmode"`
+}
+
+// DBConfig is a legacy alias for MySQLConfig
+type DBConfig = MySQLConfig
+
+// Validate ensures the configuration is valid and unambiguous
+func (c *Config) Validate() error {
+	hasMySQL := c.MySQL != nil
+	hasPostgres := c.Postgres != nil
+	hasLegacy := c.User != "" || c.Host != "" || c.Port != 0 || c.Name != ""
+
+	count := 0
+	if hasMySQL {
+		count++
+	}
+	if hasPostgres {
+		count++
+	}
+	if hasLegacy {
+		count++
+	}
+
+	if count == 0 {
+		return fmt.Errorf("no database configuration provided")
+	}
+	if count > 1 {
+		return fmt.Errorf("multiple database configurations provided (only one allowed)")
+	}
+
+	return nil
 }
 
 // ConfigOptions allows customization of database opening behavior