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/config.go

@@ -0,0 +1,61 @@
+package database
+
+import (
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Config represents the database configuration structure
+type Config struct {
+	MySQL DBConfig `yaml:"mysql"`
+}
+
+// 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
+}
+
+// ConfigOptions allows customization of database opening behavior
+type ConfigOptions struct {
+	// ConfigFiles is a list of config file paths to search for database configuration
+	ConfigFiles []string
+
+	// EnablePoolMonitoring enables connection pool metrics collection
+	EnablePoolMonitoring bool
+
+	// PrometheusRegisterer for metrics collection. If nil, no metrics are collected.
+	PrometheusRegisterer prometheus.Registerer
+
+	// Connection pool settings
+	MaxOpenConns    int
+	MaxIdleConns    int
+	ConnMaxLifetime time.Duration
+}
+
+// DefaultConfigOptions returns the standard configuration options used by API package
+func DefaultConfigOptions() ConfigOptions {
+	return ConfigOptions{
+		ConfigFiles:          []string{"database.yaml", "/vault/secrets/database.yaml"},
+		EnablePoolMonitoring: true,
+		PrometheusRegisterer: prometheus.DefaultRegisterer,
+		MaxOpenConns:         25,
+		MaxIdleConns:         10,
+		ConnMaxLifetime:      3 * time.Minute,
+	}
+}
+
+// MonitorConfigOptions returns configuration options optimized for Monitor package
+func MonitorConfigOptions() ConfigOptions {
+	return ConfigOptions{
+		ConfigFiles:          []string{"database.yaml", "/vault/secrets/database.yaml"},
+		EnablePoolMonitoring: false, // Monitor doesn't need metrics
+		PrometheusRegisterer: nil,   // No Prometheus dependency
+		MaxOpenConns:         10,
+		MaxIdleConns:         5,
+		ConnMaxLifetime:      3 * time.Minute,
+	}
+}