database: create shared database package with configurable patterns

Extract ~200 lines of duplicate database connection code from api/ntpdb/
and monitor/ntpdb/ into common/database/ package. Creates foundation for
database consolidation while maintaining zero breaking changes.

Files added:
- config.go: Unified configuration with package-specific defaults
- connector.go: Dynamic connector pattern from Boostport
- pool.go: Configurable connection pool management
- metrics.go: Optional Prometheus metrics integration
- interfaces.go: Shared database interfaces for consistent patterns

Key features:
- Configuration-driven approach (API: 25/10 connections + metrics,
  Monitor: 10/5 connections, no metrics)
- Optional Prometheus metrics when registerer provided
- Backward compatibility via convenience functions
- Flexible config file loading (explicit paths + search-based)

Dependencies: Added mysql driver and yaml parsing for database configuration.

database/pool.go

@@ -0,0 +1,78 @@
+package database
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"os"
+
+	"go.ntppool.org/common/logger"
+)
+
+// OpenDB opens a database connection with the specified configuration options
+func OpenDB(ctx context.Context, options ConfigOptions) (*sql.DB, error) {
+	log := logger.Setup()
+
+	configFile, err := findConfigFile(options.ConfigFiles)
+	if err != nil {
+		return nil, err
+	}
+
+	dbconn := sql.OpenDB(Driver{
+		CreateConnectorFunc: createConnector(configFile),
+	})
+
+	// Set connection pool parameters
+	dbconn.SetConnMaxLifetime(options.ConnMaxLifetime)
+	dbconn.SetMaxOpenConns(options.MaxOpenConns)
+	dbconn.SetMaxIdleConns(options.MaxIdleConns)
+
+	err = dbconn.Ping()
+	if err != nil {
+		log.Error("could not connect to database", "err", err)
+		return nil, err
+	}
+
+	// Start optional connection pool monitoring
+	if options.EnablePoolMonitoring && options.PrometheusRegisterer != nil {
+		go monitorConnectionPool(ctx, dbconn, options.PrometheusRegisterer)
+	}
+
+	return dbconn, nil
+}
+
+// OpenDBWithConfigFile opens a database connection using an explicit config file path
+// This is a convenience function for API package compatibility
+func OpenDBWithConfigFile(ctx context.Context, configFile string) (*sql.DB, error) {
+	options := DefaultConfigOptions()
+	options.ConfigFiles = []string{configFile}
+	return OpenDB(ctx, options)
+}
+
+// OpenDBMonitor opens a database connection with monitor-specific defaults
+// This is a convenience function for Monitor package compatibility
+func OpenDBMonitor() (*sql.DB, error) {
+	options := MonitorConfigOptions()
+	return OpenDB(context.Background(), options)
+}
+
+// findConfigFile searches for the first existing config file from the list
+func findConfigFile(configFiles []string) (string, error) {
+	var firstErr error
+
+	for _, configFile := range configFiles {
+		if configFile == "" {
+			continue
+		}
+		if _, err := os.Stat(configFile); err == nil {
+			return configFile, nil
+		} else if firstErr == nil {
+			firstErr = err
+		}
+	}
+
+	if firstErr != nil {
+		return "", fmt.Errorf("no config file found: %w", firstErr)
+	}
+	return "", fmt.Errorf("no valid config files provided")
+}