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/connector.go

@@ -0,0 +1,88 @@
+package database
+
+import (
+	"context"
+	"database/sql/driver"
+	"errors"
+	"fmt"
+	"os"
+
+	"github.com/go-sql-driver/mysql"
+	"gopkg.in/yaml.v3"
+)
+
+// from https://github.com/Boostport/dynamic-database-config
+
+// CreateConnectorFunc is a function that creates a database connector
+type CreateConnectorFunc func() (driver.Connector, error)
+
+// Driver implements the sql/driver interface with dynamic configuration
+type Driver struct {
+	CreateConnectorFunc CreateConnectorFunc
+}
+
+// Driver returns the driver instance
+func (d Driver) Driver() driver.Driver {
+	return d
+}
+
+// Connect creates a new database connection using the dynamic connector
+func (d Driver) Connect(ctx context.Context) (driver.Conn, error) {
+	connector, err := d.CreateConnectorFunc()
+	if err != nil {
+		return nil, fmt.Errorf("error creating connector from function: %w", err)
+	}
+
+	return connector.Connect(ctx)
+}
+
+// Open is not supported for dynamic configuration
+func (d Driver) Open(name string) (driver.Conn, error) {
+	return nil, errors.New("open is not supported")
+}
+
+// createConnector creates a connector function that reads configuration from a file
+func createConnector(configFile string) CreateConnectorFunc {
+	return func() (driver.Connector, error) {
+		dbFile, err := os.Open(configFile)
+		if err != nil {
+			return nil, err
+		}
+		defer dbFile.Close()
+
+		dec := yaml.NewDecoder(dbFile)
+		cfg := Config{}
+
+		err = dec.Decode(&cfg)
+		if err != nil {
+			return nil, err
+		}
+
+		dsn := cfg.MySQL.DSN
+		if len(dsn) == 0 {
+			dsn = os.Getenv("DATABASE_DSN")
+			if len(dsn) == 0 {
+				return nil, fmt.Errorf("dsn config in database.yaml or DATABASE_DSN environment variable required")
+			}
+		}
+
+		dbcfg, err := mysql.ParseDSN(dsn)
+		if err != nil {
+			return nil, err
+		}
+
+		if user := cfg.MySQL.User; len(user) > 0 {
+			dbcfg.User = user
+		}
+
+		if pass := cfg.MySQL.Pass; len(pass) > 0 {
+			dbcfg.Passwd = pass
+		}
+
+		if name := cfg.MySQL.DBName; len(name) > 0 {
+			dbcfg.DBName = name
+		}
+
+		return mysql.NewConnector(dbcfg)
+	}
+}