Add SafeApply lifecycle, health tracking, convergence loop, and startup checks

SafeApply pattern (validate → backup → write → reload → verify → rollback):
- HAProxy: SafeApply wraps existing validate+write+reload with backup and
  post-reload health check; rolls back to .bak on failure
- dnsmasq: SafeApply validates via dnsmasq --test, backs up, atomic writes,
  restarts, verifies daemon is active; rolls back on failure
- nftables: SafeApply validates via nft -c, backs up, atomic writes, applies
  to kernel, verifies table loaded; rolls back on failure

Health tracking:
- Add SubsystemHealth and Health structs to reconciler
- Track per-subsystem status (ok/degraded/error) after each reconcile
- Detect recovery: previous error → current ok broadcasts recovery event
- GET /api/v1/health/reconcile endpoint exposes health state
- HAProxy tracks excluded domains as "degraded" state

SSE error events:
- Broadcast haproxy:error, dnsmasq:error on SafeApply failure
- Broadcast haproxy:recovered, dnsmasq:recovered on recovery from error

Convergence loop:
- 5-minute periodic reconcile drives system toward desired state
- Catches config drift, daemon crashes, transient failures
- Serialized by reconcile mutex — no race with event-driven reconciles

Startup:
- CheckPrerequisites verifies required (dnsmasq, haproxy) and optional
  (wg, authelia, cscli, cloudflared, nft) binaries before first reconcile

internal/nftables/manager.go

@@ -131,6 +131,62 @@ func (m *Manager) ValidateRules(rulesPath string) error {
 	return nil
 }
 
+// SafeApply validates, backs up, writes, and applies nftables rules.
+// On apply failure, rolls back to the previous rules.
+func (m *Manager) SafeApply(content string) error {
+	// Validate syntax
+	tmpFile := "/tmp/wild-cloud-nft-safeapply.tmp"
+	if err := os.WriteFile(tmpFile, []byte(content), 0644); err != nil {
+		return fmt.Errorf("writing validation file: %w", err)
+	}
+	defer os.Remove(tmpFile)
+
+	if err := m.ValidateRules(tmpFile); err != nil {
+		return fmt.Errorf("validation failed: %w", err)
+	}
+
+	// Backup current rules
+	if storage.FileExists(m.rulesPath) {
+		_ = storage.CopyFile(m.rulesPath, m.rulesPath+".bak")
+	}
+
+	// Write atomically
+	if err := storage.WriteFileAtomic(m.rulesPath, []byte(content), 0644); err != nil {
+		return fmt.Errorf("write failed: %w", err)
+	}
+
+	// Apply to kernel
+	if err := m.ApplyRules(); err != nil {
+		m.rollback()
+		return fmt.Errorf("apply failed (rolled back): %w", err)
+	}
+
+	// Verify rules loaded
+	if err := m.verify(); err != nil {
+		m.rollback()
+		return fmt.Errorf("verification failed (rolled back): %w", err)
+	}
+
+	return nil
+}
+
+func (m *Manager) rollback() {
+	bakPath := m.rulesPath + ".bak"
+	if storage.FileExists(bakPath) {
+		_ = os.Rename(bakPath, m.rulesPath)
+		_ = m.ApplyRules()
+		slog.Warn("rolled back to previous rules", "component", "nftables", "path", m.rulesPath)
+	}
+}
+
+func (m *Manager) verify() error {
+	status, err := m.GetStatus()
+	if err != nil || status == "" {
+		return fmt.Errorf("nftables table not loaded after apply")
+	}
+	return nil
+}
+
 // WriteRules validates the content then writes the nftables rules file.
 // Validation uses a temp file in /tmp (world-writable) to avoid needing
 // write permission on the /etc/nftables.d/ directory itself.