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/store/store.go

@@ -21,6 +21,12 @@ type Store interface {
 	UpdateAlert(id int, name, aType string, settings map[string]string) error
 	DeleteAlert(id int) error
 
+	// Declarative config support
+	GetSiteByName(name string) (models.Site, error)
+	GetAlertByName(name string) (models.AlertConfig, error)
+	AddSiteReturningID(site models.Site) (int, error)
+	AddAlertReturningID(name, aType string, settings map[string]string) (int, error)
+
 	// Users
 	GetAllUsers() ([]models.User, error)
 	AddUser(username, publicKey, role string) error