database: create shared database package

Extract common database functionality from api/ntpdb and monitor/ntpdb
into shared common/database package:

- Dynamic connector pattern with configuration loading
- Configurable connection pool management (API: 25/10, Monitor: 10/5)
- Optional Prometheus metrics integration
- Generic transaction helpers with proper error handling
- Unified interfaces compatible with SQLC-generated code

Foundation for migration to eliminate ~200 lines of duplicate code.

database/config_test.go

@@ -0,0 +1,81 @@
+package database
+
+import (
+	"testing"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+func TestDefaultConfigOptions(t *testing.T) {
+	opts := DefaultConfigOptions()
+
+	// Verify expected defaults for API package
+	if opts.MaxOpenConns != 25 {
+		t.Errorf("Expected MaxOpenConns=25, got %d", opts.MaxOpenConns)
+	}
+	if opts.MaxIdleConns != 10 {
+		t.Errorf("Expected MaxIdleConns=10, got %d", opts.MaxIdleConns)
+	}
+	if opts.ConnMaxLifetime != 3*time.Minute {
+		t.Errorf("Expected ConnMaxLifetime=3m, got %v", opts.ConnMaxLifetime)
+	}
+	if !opts.EnablePoolMonitoring {
+		t.Error("Expected EnablePoolMonitoring=true")
+	}
+	if opts.PrometheusRegisterer != prometheus.DefaultRegisterer {
+		t.Error("Expected PrometheusRegisterer to be DefaultRegisterer")
+	}
+	if len(opts.ConfigFiles) == 0 {
+		t.Error("Expected ConfigFiles to be non-empty")
+	}
+}
+
+func TestMonitorConfigOptions(t *testing.T) {
+	opts := MonitorConfigOptions()
+
+	// Verify expected defaults for Monitor package
+	if opts.MaxOpenConns != 10 {
+		t.Errorf("Expected MaxOpenConns=10, got %d", opts.MaxOpenConns)
+	}
+	if opts.MaxIdleConns != 5 {
+		t.Errorf("Expected MaxIdleConns=5, got %d", opts.MaxIdleConns)
+	}
+	if opts.ConnMaxLifetime != 3*time.Minute {
+		t.Errorf("Expected ConnMaxLifetime=3m, got %v", opts.ConnMaxLifetime)
+	}
+	if opts.EnablePoolMonitoring {
+		t.Error("Expected EnablePoolMonitoring=false")
+	}
+	if opts.PrometheusRegisterer != nil {
+		t.Error("Expected PrometheusRegisterer to be nil")
+	}
+	if len(opts.ConfigFiles) == 0 {
+		t.Error("Expected ConfigFiles to be non-empty")
+	}
+}
+
+func TestConfigStructures(t *testing.T) {
+	// Test that configuration structures can be created and populated
+	config := Config{
+		MySQL: DBConfig{
+			DSN:    "user:pass@tcp(localhost:3306)/dbname",
+			User:   "testuser",
+			Pass:   "testpass",
+			DBName: "testdb",
+		},
+	}
+
+	if config.MySQL.DSN == "" {
+		t.Error("Expected DSN to be set")
+	}
+	if config.MySQL.User != "testuser" {
+		t.Errorf("Expected User='testuser', got '%s'", config.MySQL.User)
+	}
+	if config.MySQL.Pass != "testpass" {
+		t.Errorf("Expected Pass='testpass', got '%s'", config.MySQL.Pass)
+	}
+	if config.MySQL.DBName != "testdb" {
+		t.Errorf("Expected DBName='testdb', got '%s'", config.MySQL.DBName)
+	}
+}