feat(config): add config-as-code YAML import/export

Add declarative config-as-code support via YAML files. Monitors and
alerts can be exported, version controlled, and applied across instances.

- goupkeep export [-o file.yaml] dumps current state
- goupkeep apply -f file.yaml creates/updates to match desired state
- --dry-run shows planned changes without applying
- --prune deletes monitors/alerts not in the YAML
- Matching by name, alert references by name, nested group children
- CLI refactored to subcommands (apply, export, serve) with backward compat
- 24 tests covering apply, export, validation, round-trip idempotency

internal/config/export.go

@@ -0,0 +1,154 @@
+package config
+
+import (
+	"fmt"
+	"go-upkeep/internal/models"
+	"go-upkeep/internal/store"
+	"os"
+	"sort"
+
+	"gopkg.in/yaml.v3"
+)
+
+func Export(s store.Store) (*File, error) {
+	dbAlerts, err := s.GetAllAlerts()
+	if err != nil {
+		return nil, fmt.Errorf("load alerts: %w", err)
+	}
+
+	dbSites, err := s.GetSites()
+	if err != nil {
+		return nil, fmt.Errorf("load sites: %w", err)
+	}
+
+	alertIDToName := make(map[int]string, len(dbAlerts))
+	var yamlAlerts []Alert
+	for _, a := range dbAlerts {
+		alertIDToName[a.ID] = a.Name
+		yamlAlerts = append(yamlAlerts, Alert{
+			Name:     a.Name,
+			Type:     a.Type,
+			Settings: a.Settings,
+		})
+	}
+
+	groups := make(map[int]models.Site)
+	children := make(map[int][]models.Site)
+	var topLevel []models.Site
+
+	for _, s := range dbSites {
+		switch {
+		case s.Type == "group":
+			groups[s.ID] = s
+		case s.ParentID > 0:
+			children[s.ParentID] = append(children[s.ParentID], s)
+		default:
+			topLevel = append(topLevel, s)
+		}
+	}
+
+	var yamlMonitors []Monitor
+
+	groupIDs := make([]int, 0, len(groups))
+	for id := range groups {
+		groupIDs = append(groupIDs, id)
+	}
+	sort.Ints(groupIDs)
+
+	for _, gid := range groupIDs {
+		g := groups[gid]
+		ym := siteToMonitor(g, alertIDToName)
+		kids := children[gid]
+		sort.Slice(kids, func(i, j int) bool { return kids[i].ID < kids[j].ID })
+		for _, child := range kids {
+			ym.Monitors = append(ym.Monitors, siteToMonitor(child, alertIDToName))
+		}
+		yamlMonitors = append(yamlMonitors, ym)
+	}
+
+	sort.Slice(topLevel, func(i, j int) bool { return topLevel[i].ID < topLevel[j].ID })
+	for _, s := range topLevel {
+		yamlMonitors = append(yamlMonitors, siteToMonitor(s, alertIDToName))
+	}
+
+	return &File{Alerts: yamlAlerts, Monitors: yamlMonitors}, nil
+}
+
+func siteToMonitor(s models.Site, alertIDToName map[int]string) Monitor {
+	m := Monitor{
+		Name:     s.Name,
+		Type:     s.Type,
+		Interval: s.Interval,
+	}
+
+	if s.AlertID > 0 {
+		if name, ok := alertIDToName[s.AlertID]; ok {
+			m.Alert = name
+		}
+	}
+
+	if s.URL != "" {
+		m.URL = s.URL
+	}
+	if s.Hostname != "" {
+		m.Hostname = s.Hostname
+	}
+	if s.Port != 0 {
+		m.Port = s.Port
+	}
+	if s.Timeout != 0 {
+		m.Timeout = s.Timeout
+	}
+	if s.Description != "" {
+		m.Description = s.Description
+	}
+	if s.DNSResolveType != "" {
+		m.DNSResolveType = s.DNSResolveType
+	}
+	if s.DNSServer != "" {
+		m.DNSServer = s.DNSServer
+	}
+
+	if s.Method != "" && s.Method != "GET" {
+		m.Method = s.Method
+	}
+	if s.AcceptedCodes != "" && s.AcceptedCodes != "200-299" {
+		m.AcceptedCodes = s.AcceptedCodes
+	}
+	if s.ExpiryThreshold != 0 && s.ExpiryThreshold != 7 {
+		m.ExpiryThreshold = s.ExpiryThreshold
+	}
+	if s.MaxRetries != 0 {
+		m.MaxRetries = s.MaxRetries
+	}
+
+	m.CheckSSL = s.CheckSSL
+	m.IgnoreTLS = s.IgnoreTLS
+	m.Paused = s.Paused
+
+	return m
+}
+
+func WriteFile(f *File, path string) error {
+	data, err := yaml.Marshal(f)
+	if err != nil {
+		return fmt.Errorf("marshal yaml: %w", err)
+	}
+	if path == "-" || path == "" {
+		_, err = os.Stdout.Write(data)
+		return err
+	}
+	return os.WriteFile(path, data, 0644)
+}
+
+func LoadFile(path string) (*File, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("read %s: %w", path, err)
+	}
+	var f File
+	if err := yaml.Unmarshal(data, &f); err != nil {
+		return nil, fmt.Errorf("parse %s: %w", path, err)
+	}
+	return &f, nil
+}