slice 8B (controller half): app-consistent backup quiesce loop (v0.36.0)

internal/quiesce: poll /backup/due -> quiesce (stop app stacks) -> POST /backup
-> poll /backup/status -> unquiesce (restart exactly those). Crash-safety:
persisted marker before stopping, guaranteed unquiesce (defer), max-quiesce
guard, startup Recover, single-flight. agentapi BackupDue/StartBackup/
BackupStatus; stacks.RunningAppStacks(); config QuiesceConfig; main wiring.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

controller/internal/config/config.go

@@ -29,6 +29,7 @@ type Config struct {
 	Assets         AssetsConfig         `yaml:"assets"`
 	System         SystemConfig         `yaml:"system"`
 	LocalAPI       LocalAPIConfig       `yaml:"local_api"`
+	Quiesce        QuiesceConfig        `yaml:"quiesce"`
 }
 
 // LocalAPIConfig is the in-guest controller's handle on the host agent's per-guest local API
@@ -40,6 +41,22 @@ type LocalAPIConfig struct {
 	Token       string `yaml:"token"`       // per-guest bearer; SECRET
 }
 
+// QuiesceConfig tunes the slice-8B app-consistent backup loop (doc 03 §8): poll the agent's
+// /backup/due, and when due stop the app stacks around the agent vzdump for a clean-shutdown
+// backup. Runs only when the local API is configured (a provisioned guest). MaxQuiesce bounds the
+// app downtime — the controller unquiesces no matter what once it elapses.
+type QuiesceConfig struct {
+	Enabled    *bool  `yaml:"enabled"`      // nil/true → on when local API configured; false → off
+	PollInterval string `yaml:"poll_interval"`        // /backup/due check cadence (default "5m")
+	StatusPoll   string `yaml:"status_poll_interval"` // /backup/status poll while quiesced (default "10s")
+	MaxQuiesce   string `yaml:"max_quiesce_duration"` // hard downtime bound (default "30m")
+}
+
+// QuiesceEnabled reports whether the quiesce loop should run (default on unless explicitly false).
+func (q QuiesceConfig) QuiesceEnabled() bool {
+	return q.Enabled == nil || *q.Enabled
+}
+
 type SystemConfig struct {
 	ReservedMemoryMB int `yaml:"reserved_memory_mb"`
 }
@@ -278,6 +295,9 @@ func applyDefaults(cfg *Config) {
 	d(&cfg.Assets.SourceURL, "https://felhom.eu")
 	d(&cfg.Assets.SyncSchedule, "05:00")
 	di(&cfg.System.ReservedMemoryMB, 384)
+	d(&cfg.Quiesce.PollInterval, "5m")
+	d(&cfg.Quiesce.StatusPoll, "10s")
+	d(&cfg.Quiesce.MaxQuiesce, "30m")
 }
 
 func applyEnvOverrides(cfg *Config) {