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

go.mod

@@ -5,6 +5,7 @@ go 1.23.5
 require (
 	github.com/abh/certman v0.4.0
 	github.com/go-sql-driver/mysql v1.9.3
+	github.com/jackc/pgx/v5 v5.7.6
 	github.com/labstack/echo-contrib v0.17.2
 	github.com/labstack/echo/v4 v4.13.3
 	github.com/oklog/ulid/v2 v2.1.0
@@ -34,7 +35,7 @@ require (
 	go.opentelemetry.io/otel/trace v1.33.0
 	golang.org/x/mod v0.22.0
 	golang.org/x/net v0.33.0
-	golang.org/x/sync v0.10.0
+	golang.org/x/sync v0.13.0
 	google.golang.org/grpc v1.69.2
 	gopkg.in/yaml.v3 v3.0.1
 )
@@ -50,6 +51,9 @@ require (
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/grpc-ecosystem/grpc-gateway/v2 v2.25.1 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/jackc/pgpassfile v1.0.0 // indirect
+	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
+	github.com/jackc/puddle/v2 v2.2.2 // indirect
 	github.com/klauspost/compress v1.17.11 // indirect
 	github.com/labstack/gommon v0.4.2 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
@@ -70,9 +74,9 @@ require (
 	go.opentelemetry.io/otel/exporters/stdout/stdoutmetric v1.33.0 // indirect
 	go.opentelemetry.io/otel/exporters/stdout/stdouttrace v1.33.0 // indirect
 	go.opentelemetry.io/proto/otlp v1.4.0 // indirect
-	golang.org/x/crypto v0.31.0 // indirect
-	golang.org/x/sys v0.28.0 // indirect
-	golang.org/x/text v0.21.0 // indirect
+	golang.org/x/crypto v0.37.0 // indirect
+	golang.org/x/sys v0.32.0 // indirect
+	golang.org/x/text v0.24.0 // indirect
 	golang.org/x/time v0.8.0 // indirect
 	google.golang.org/genproto/googleapis/api v0.0.0-20241223144023-3abc09e42ca8 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20241223144023-3abc09e42ca8 // indirect