v0.4.0: monitoring & backup — scheduler, CPU/temp metrics, healthchecks, restic backups

Phase 2 (Monitoring & Health):
- Central job scheduler replacing ad-hoc goroutines (internal/scheduler)
- CPU usage collector via /proc/stat background sampling (internal/system/cpu_linux.go)
- Temperature reading from /sys/class/thermal + /host/sys (Docker mount)
- Load average from /proc/loadavg
- Healthchecks.io-compatible HTTP pinger (internal/monitor/pinger.go)
- System health checks: disk, memory, CPU, temp, Docker, protected containers (internal/monitor/healthcheck.go)

Phase 3 (Backups):
- Database auto-discovery via docker ps + docker inspect (internal/backup/dbdump.go)
- Database dumping via docker exec (pg_dump / mariadb-dump) with atomic writes
- Restic backup integration with auto-password generation (internal/backup/restic.go)
- Backup orchestrator: DB dumps + restic snapshots + weekly prune (internal/backup/backup.go)
- Manual backup trigger via dashboard button and POST /api/backup/run

Dashboard UI:
- CPU usage bar with load average display
- Temperature with colored indicator dot
- Backup status card with last run time, DB count, repo stats
- "Mentés most" button for manual backup trigger

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

controller/internal/system/cpu_other.go

@@ -0,0 +1,25 @@
+//go:build !linux
+
+package system
+
+import (
+	"context"
+	"time"
+)
+
+// CPUCollector is a no-op on non-Linux platforms.
+type CPUCollector struct{}
+
+// NewCPUCollector creates a no-op CPU collector on non-Linux platforms.
+func NewCPUCollector(_ time.Duration) *CPUCollector {
+	return &CPUCollector{}
+}
+
+// Start is a no-op on non-Linux platforms.
+func (c *CPUCollector) Start(_ context.Context) {}
+
+// Stop is a no-op on non-Linux platforms.
+func (c *CPUCollector) Stop() {}
+
+// CPUPercent always returns 0 on non-Linux platforms.
+func (c *CPUCollector) CPUPercent() float64 { return 0 }