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/integration_test.go

@@ -0,0 +1,117 @@
+package database
+
+import (
+	"context"
+	"database/sql"
+	"testing"
+)
+
+// Mock types for testing SQLC integration patterns
+type mockQueries struct {
+	db DBTX
+}
+
+type mockQueriesTx struct {
+	*mockQueries
+	tx *sql.Tx
+}
+
+// Mock the Begin method pattern that SQLC generates
+func (q *mockQueries) Begin(ctx context.Context) (*mockQueriesTx, error) {
+	// This would normally be: tx, err := q.db.(*sql.DB).BeginTx(ctx, nil)
+	// For our test, we return a mock
+	return &mockQueriesTx{mockQueries: q, tx: nil}, nil
+}
+
+func (qtx *mockQueriesTx) Commit(ctx context.Context) error {
+	return nil // Mock implementation
+}
+
+func (qtx *mockQueriesTx) Rollback(ctx context.Context) error {
+	return nil // Mock implementation
+}
+
+// This test verifies that our common database interfaces are compatible with SQLC-generated code
+func TestSQLCIntegration(t *testing.T) {
+	// Test that SQLC's DBTX interface matches our DBTX interface
+	t.Run("DBTX Interface Compatibility", func(t *testing.T) {
+		// Test interface compatibility by assignment without execution
+		var ourDBTX DBTX
+
+		// Test with sql.DB (should implement DBTX)
+		var db *sql.DB
+		ourDBTX = db // This will compile only if interfaces are compatible
+		_ = ourDBTX  // Use the variable to avoid "unused" warning
+
+		// Test with sql.Tx (should implement DBTX)
+		var tx *sql.Tx
+		ourDBTX = tx // This will compile only if interfaces are compatible
+		_ = ourDBTX  // Use the variable to avoid "unused" warning
+
+		// If we reach here, interfaces are compatible
+		t.Log("DBTX interface is compatible with sql.DB and sql.Tx")
+	})
+
+	t.Run("Transaction Interface Compatibility", func(t *testing.T) {
+		// This test verifies our transaction interfaces work with SQLC patterns
+		// We can't define methods inside a function, so we test interface compatibility
+
+		// Verify our DB interface is compatible with what SQLC expects
+		var dbInterface DB[*mockQueriesTx]
+		var mockDB *mockQueries = &mockQueries{}
+		dbInterface = mockDB
+
+		// Test that our transaction helper can work with this pattern
+		err := WithTransaction(context.Background(), dbInterface, func(ctx context.Context, qtx *mockQueriesTx) error {
+			// This would be where you'd call SQLC-generated query methods
+			return nil
+		})
+		if err != nil {
+			t.Errorf("Transaction helper failed: %v", err)
+		}
+	})
+}
+
+// Test that demonstrates how the common package would be used with real SQLC patterns
+func TestRealWorldUsagePattern(t *testing.T) {
+	// This test shows how a package would typically use our common database code
+
+	t.Run("Database Opening Pattern", func(t *testing.T) {
+		// Test that our configuration options work as expected
+		opts := DefaultConfigOptions()
+
+		// Modify for test environment (no actual database connection)
+		opts.ConfigFiles = []string{}   // No config files for unit test
+		opts.PrometheusRegisterer = nil // No metrics for unit test
+
+		// This would normally open a database: db, err := OpenDB(ctx, opts)
+		// For our unit test, we just verify the options are reasonable
+		if opts.MaxOpenConns <= 0 {
+			t.Error("MaxOpenConns should be positive")
+		}
+		if opts.MaxIdleConns <= 0 {
+			t.Error("MaxIdleConns should be positive")
+		}
+		if opts.ConnMaxLifetime <= 0 {
+			t.Error("ConnMaxLifetime should be positive")
+		}
+	})
+
+	t.Run("Monitor Package Configuration", func(t *testing.T) {
+		opts := MonitorConfigOptions()
+
+		// Verify monitor-specific settings
+		if opts.EnablePoolMonitoring {
+			t.Error("Monitor package should not enable pool monitoring")
+		}
+		if opts.PrometheusRegisterer != nil {
+			t.Error("Monitor package should not have Prometheus registerer")
+		}
+		if opts.MaxOpenConns != 10 {
+			t.Errorf("Expected MaxOpenConns=10 for monitor, got %d", opts.MaxOpenConns)
+		}
+		if opts.MaxIdleConns != 5 {
+			t.Errorf("Expected MaxIdleConns=5 for monitor, got %d", opts.MaxIdleConns)
+		}
+	})
+}