fix(filter): suppress subscription renewal in background mode

Adds backgroundMode to Sub and SetBackgroundMode(bool) to both Sub and
FilterManager. When background=true, subscriptionLoop skips the 5-second
health-check ticker and drops expired subscription IDs from the closing
channel without resubscribing. When background=false (foreground return),
a resubscription is immediately triggered for any expired filters.

Background context (status-im/status-app#21045):
On Android, each Waku filter subscription has a relay-side TTL (~13.5 min
observed). When it expires, the closing channel fires, checkAndResubscribe
runs, and a new wf.Subscribe() RPC wakes the LTE modem. With a loaded
account this happens every ~13.5 min overnight, producing a 55% radio
duty cycle (~144 mAh/hr) while the screen is locked.

With background mode active, no network I/O occurs during subscription
expiry. On foreground return, all expired filters are resubscribed in
one burst — the user sees a brief reconnect, then full message delivery.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

waku/v2/api/filter/filter.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/google/uuid"
@@ -57,6 +58,9 @@ type Sub struct {
 	resubscribeInProgress bool
 	id                    string
 	errcnt                int
+	// backgroundMode suppresses subscription renewal when the app UI is not visible.
+	// Toggled via SetBackgroundMode; read from subscriptionLoop goroutine.
+	backgroundMode atomic.Bool
 	// rateLimitedUntil is set when subscribe() observes a *SubscribeError whose
 	// FailedPeers contain at least one HTTP 429. While time.Now().Before(rateLimitedUntil),
 	// subscriptionLoop suppresses retry triggers (ticker push and checkAndResubscribe).
@@ -139,6 +143,11 @@ func (apiSub *Sub) subscriptionLoop(loopInterval time.Duration) {
 		select {
 		case <-ticker.C:
 			apiSub.errcnt = 0 //reset errorCount
+			if apiSub.backgroundMode.Load() {
+				// In background: skip health check to avoid waking the LTE radio.
+				// SetBackgroundMode(false) triggers resubscription on foreground.
+				continue
+			}
 			if shouldHonourRateLimitBackoff(apiSub.rateLimitedUntil, time.Now()) {
 				apiSub.log.Debug("ticker push suppressed by rate-limit backoff",
 					zap.Time("rate-limited-until", apiSub.rateLimitedUntil),
@@ -154,6 +163,14 @@ func (apiSub *Sub) subscriptionLoop(loopInterval time.Duration) {
 			apiSub.cleanup()
 			return
 		case subId := <-apiSub.closing:
+			if apiSub.backgroundMode.Load() {
+				// In background: subscription expired but don't resubscribe now.
+				// SetBackgroundMode(false) will trigger resubscription on foreground.
+				apiSub.log.Debug("resubscribe suppressed: app in background",
+					zap.String("sub-id", subId),
+				)
+				continue
+			}
 			if shouldHonourRateLimitBackoff(apiSub.rateLimitedUntil, time.Now()) {
 				apiSub.log.Debug("checkAndResubscribe suppressed by rate-limit backoff",
 					zap.Time("rate-limited-until", apiSub.rateLimitedUntil),
@@ -174,6 +191,23 @@ func (apiSub *Sub) subscriptionLoop(loopInterval time.Duration) {
 	}
 }
 
+// SetBackgroundMode controls whether this subscription suppresses renewal.
+// Call with background=true when the app UI is not visible (screen locked).
+// Call with background=false when returning to foreground; this triggers an
+// immediate resubscription attempt for any subscriptions that expired while
+// backgrounded.
+func (apiSub *Sub) SetBackgroundMode(background bool) {
+	apiSub.backgroundMode.Store(background)
+	if !background {
+		// Returning to foreground: prod the loop to resubscribe.
+		select {
+		case apiSub.closing <- "":
+		default:
+			// A resubscription is already queued.
+		}
+	}
+}
+
 func (apiSub *Sub) checkAndResubscribe(subId string) {
 	var failedPeer peer.ID
 	if subId != "" {

waku/v2/api/filter/filter_manager.go

@@ -47,9 +47,9 @@ type FilterManager struct {
 	// a deadlock where SubscribeFilter would block on a full channel while still
 	// holding mgr.Lock(), preventing the only drainer (checkAndProcessQueue, also
 	// invoked under the same lock) from running.
-	waitingToSubQueue      []filterConfig
-	envProcessor           EnevelopeProcessor
-	networkConnType        byte
+	waitingToSubQueue []filterConfig
+	envProcessor      EnevelopeProcessor
+	networkConnType   byte
 }
 
 type SubDetails struct {
@@ -189,6 +189,19 @@ func (mgr *FilterManager) NetworkChange() {
 	mgr.node.PingPeers() // ping all peers to check if subscriptions are alive
 }
 
+// SetBackgroundMode notifies all active subscriptions of the app's visibility
+// state. When background=true, subscriptions suppress renewal keepalives to
+// avoid waking the LTE radio while the screen is locked. When background=false
+// (returning to foreground), each subscription immediately attempts to
+// resubscribe if its filter has expired.
+func (mgr *FilterManager) SetBackgroundMode(background bool) {
+	mgr.Lock()
+	defer mgr.Unlock()
+	for _, subDetails := range mgr.filterSubscriptions {
+		subDetails.sub.SetBackgroundMode(background)
+	}
+}
+
 // checkAndProcessQueue drains the offline-pending filter queue. For each batch
 // that matches the given pubsubTopic (or always, when pubsubTopic == ""), a
 // subscribe goroutine is spawned; non-matching batches are retained for a