feat: comprehensive debug logging across all controller modules

Add detailed [DEBUG] logging to every controller module when
logging.level is set to "debug". Each module with stateful debug
uses SetDebug(bool) wired from main.go. Covers stacks, backup,
cloudflare, integrations, system, monitor, settings, scheduler,
web handlers, storage, metrics, API, selfupdate, and assets.

Also includes the app export/import (.fab bundles) feature from
v0.32.0 and its debug page integration.

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

controller/internal/system/cpu_linux.go

@@ -30,6 +30,7 @@ func NewCPUCollector(sampleRate time.Duration) *CPUCollector {
 // Start begins background CPU sampling.
 func (c *CPUCollector) Start(ctx context.Context) {
 	ctx, c.cancel = context.WithCancel(ctx)
+	debugf("[DEBUG] [system] CPUCollector.Start: sampleRate=%s", c.sampleRate)
 	go c.loop(ctx)
 }
 
@@ -48,10 +49,12 @@ func (c *CPUCollector) CPUPercent() float64 {
 }
 
 func (c *CPUCollector) loop(ctx context.Context) {
+	firstSample := true
 	for {
 		// Read first sample
 		idle1, total1, err := readCPUStat()
 		if err != nil {
+			debugf("[DEBUG] [system] CPUCollector: readCPUStat error: %v", err)
 			select {
 			case <-ctx.Done():
 				return
@@ -82,6 +85,11 @@ func (c *CPUCollector) loop(ctx context.Context) {
 			c.mu.Lock()
 			c.cpuPercent = percent
 			c.mu.Unlock()
+
+			if firstSample {
+				debugf("[DEBUG] [system] CPUCollector: first sample — cpu=%.1f%% (idle=%d total=%d)", percent, idleDelta, totalDelta)
+				firstSample = false
+			}
 		}
 	}
 }