feat(catchup): arm on hello, fire missed-window backups on tick

internal/server/http/server.go

@@ -90,6 +90,13 @@ type Server struct {
 	// directories (P3-X2). Pre-allocated in New so the lazy-init
 	// race is impossible.
 	treeCache *treeCache
+
+	// catchupDueAt tracks intermittent hosts that reconnected and are
+	// in their settle window. Keyed hostID → earliest time to evaluate
+	// catch-up. Best-effort + in-memory: a server restart simply re-arms
+	// on the next hello. Guarded by catchupMu.
+	catchupMu    sync.Mutex
+	catchupDueAt map[string]time.Time
 }
 
 // New builds a configured but not-yet-started server.
@@ -104,11 +111,12 @@ func New(deps Deps) *Server {
 	r.Use(requestLogger)
 
 	s := &Server{
-		deps:       deps,
-		drainLocks: make(map[string]*sync.Mutex),
-		announceRL: newAnnounceLimiter(),
-		pendingHub: newPendingHub(),
-		treeCache:  newTreeCache(),
+		deps:         deps,
+		drainLocks:   make(map[string]*sync.Mutex),
+		announceRL:   newAnnounceLimiter(),
+		pendingHub:   newPendingHub(),
+		treeCache:    newTreeCache(),
+		catchupDueAt: make(map[string]time.Time),
 	}
 	s.routes(r)