Add monitor-only mode: suppress all actions without stopping EMS

New internal/monitor package provides a thread-safe, file-backed toggle.
Flag file presence (/etc/ems/monitor-only) = monitor-only active, survives
reboots. Deletion = resume normal operation.

Behaviour when active:
- EMS continues polling Prometheus, running the decision engine, and
  updating the status page every 2 minutes — full visibility maintained
- All actuator calls suppressed (Shelly switches + Viessmann WW writes)
- Suppressed actions logged as [MONITOR-ONLY] for audit trail
- Startup warns if flag file is already present

Web UI:
- Amber banner "⏸ Monitor-Only — Keine Schaltvorgänge" with inline
  "▶ Automatik" resume button when active
- Small unobtrusive "⏸ Monitor-Only" button at page bottom when inactive
- POST /monitor endpoint toggles state and redirects back to status page

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

internal/config/config.go

@@ -159,6 +159,7 @@ type EMSConfig struct {
 	RecoveryTimeout    string  `yaml:"recovery_timeout"`
 	OverrideTimeout    string  `yaml:"override_timeout"`
 	OverrideMaxImportW float64 `yaml:"override_max_import_w"` // cancel override if grid import exceeds this (0 = disabled)
+	MonitorOnlyFile    string  `yaml:"monitor_only_file"`     // flag file path: presence = monitor-only mode active
 }
 
 func (e *EMSConfig) PollIntervalParsed() time.Duration {

internal/monitor/mode.go

@@ -0,0 +1,61 @@
+// Package monitor provides a persistent monitor-only mode toggle.
+// When active, the EMS continues polling and showing the status page
+// but suppresses all actuator calls (Shelly switches, Viessmann API).
+// State is stored as a flag file: presence = active, absence = normal.
+package monitor
+
+import (
+	"os"
+	"sync"
+)
+
+// Mode is a thread-safe, file-backed monitor-only flag.
+type Mode struct {
+	mu       sync.RWMutex
+	active   bool
+	filePath string
+}
+
+// New creates a Mode and loads the current state from the flag file.
+// If filePath is empty, the mode is in-memory only (not persistent).
+func New(filePath string) *Mode {
+	m := &Mode{filePath: filePath}
+	if filePath != "" {
+		_, err := os.Stat(filePath)
+		m.active = err == nil // file exists → monitor-only
+	}
+	return m
+}
+
+// IsActive reports whether monitor-only mode is currently enabled.
+func (m *Mode) IsActive() bool {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	return m.active
+}
+
+// Set enables or disables monitor-only mode and persists the change to disk.
+func (m *Mode) Set(active bool) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	m.active = active
+
+	if m.filePath == "" {
+		return nil // in-memory only
+	}
+
+	if active {
+		f, err := os.Create(m.filePath)
+		if err != nil {
+			return err
+		}
+		return f.Close()
+	}
+
+	err := os.Remove(m.filePath)
+	if os.IsNotExist(err) {
+		return nil
+	}
+	return err
+}

internal/status/status.go

@@ -11,6 +11,7 @@ import (
 	"github.com/tb/ems/internal/config"
 	"github.com/tb/ems/internal/engine"
 	"github.com/tb/ems/internal/forecast"
+	"github.com/tb/ems/internal/monitor"
 )
 
 // consumerMeta holds static display info for each consumer.
@@ -50,10 +51,11 @@ type Store struct {
 	strategic    config.StrategicConfig
 	consumers    map[engine.Consumer]*consumerRecord
 	fcResult     *forecast.Result
+	monitorMode  *monitor.Mode
 }
 
 // NewStore creates a new status store.
-func NewStore(dryRun bool, wwConfigured bool, strategic config.StrategicConfig) *Store {
+func NewStore(dryRun bool, wwConfigured bool, strategic config.StrategicConfig, mode *monitor.Mode) *Store {
 	consumers := make(map[engine.Consumer]*consumerRecord, len(consumerOrder))
 	for _, c := range consumerOrder {
 		consumers[c] = &consumerRecord{}
@@ -63,6 +65,7 @@ func NewStore(dryRun bool, wwConfigured bool, strategic config.StrategicConfig)
 		wwConfigured: wwConfigured,
 		strategic:    strategic,
 		consumers:    consumers,
+		monitorMode:  mode,
 	}
 }
 
@@ -148,6 +151,7 @@ type pageData struct {
 	LastUpdate       time.Time
 	ErrMsg           string
 	DryRun           bool
+	MonitorOnly      bool
 	BatterySOC       float64
 	GridPowerW       float64
 	PVProductionW    float64
@@ -227,6 +231,8 @@ func (s *Store) Handler() http.HandlerFunc {
 			return
 		}
 
+		monitorOnly := s.monitorMode != nil && s.monitorMode.IsActive()
+
 		s.mu.RLock()
 		l1, l2, l3 := s.state.PhaseL1PowerW, s.state.PhaseL2PowerW, s.state.PhaseL3PowerW
 		hasPhase := l1 != 0 || l2 != 0 || l3 != 0
@@ -234,6 +240,7 @@ func (s *Store) Handler() http.HandlerFunc {
 			LastUpdate:       s.lastUpdate,
 			ErrMsg:           s.errMsg,
 			DryRun:           s.dryRun,
+			MonitorOnly:      monitorOnly,
 			BatterySOC:       s.state.BatterySOC,
 			GridPowerW:       s.state.GridPowerW,
 			PVProductionW:    s.state.PVProductionW,
@@ -366,8 +373,27 @@ h1 { font-size: 1.4rem; font-weight: 700; color: #14532d; }
   font-size: 0.85rem;
   margin-bottom: 1rem;
 }
-.banner.warn  { background: #fef9c3; color: #854d0e; border: 1px solid #fde68a; }
-.banner.error { background: #fee2e2; color: #991b1b; border: 1px solid #fca5a5; }
+.banner.warn    { background: #fef9c3; color: #854d0e; border: 1px solid #fde68a; }
+.banner.error   { background: #fee2e2; color: #991b1b; border: 1px solid #fca5a5; }
+.banner.monitor {
+  background: #fff7ed; color: #9a3412; border: 1px solid #fed7aa;
+  display: flex; align-items: center; justify-content: space-between; gap: 0.75rem;
+}
+.resume-btn {
+  background: #fff; border: 1px solid #9a3412; color: #9a3412;
+  border-radius: 7px; padding: 0.25rem 0.7rem; font-size: 0.8rem;
+  font-weight: 700; cursor: pointer; white-space: nowrap; flex-shrink: 0;
+}
+.resume-btn:active { opacity: 0.7; }
+.monitor-toggle {
+  text-align: center; margin-top: 1.5rem; padding-bottom: 0.5rem;
+}
+.monitor-toggle-btn {
+  background: none; border: 1px solid #d1d5db; color: #9ca3af;
+  border-radius: 8px; padding: 0.35rem 1rem; font-size: 0.75rem;
+  cursor: pointer;
+}
+.monitor-toggle-btn:hover { border-color: #9ca3af; color: #6b7280; }
 
 .grid {
   display: grid;
@@ -584,7 +610,14 @@ h1 { font-size: 1.4rem; font-weight: 700; color: #14532d; }
   {{end}}
 </header>
 
-{{if .DryRun}}
+{{if .MonitorOnly}}
+<div class="banner monitor">
+  <span>⏸ Monitor-Only — Keine Schaltvorgänge</span>
+  <form method="post" action="/monitor">
+    <button type="submit" class="resume-btn">▶ Automatik</button>
+  </form>
+</div>
+{{else if .DryRun}}
 <div class="banner warn">⚠️ Testmodus — keine echten Schaltvorgänge</div>
 {{end}}
 {{if .ErrMsg}}
@@ -729,5 +762,13 @@ h1 { font-size: 1.4rem; font-weight: 700; color: #14532d; }
 </div>
 {{end}}
 
+{{if not .MonitorOnly}}
+<div class="monitor-toggle">
+  <form method="post" action="/monitor">
+    <button type="submit" class="monitor-toggle-btn">⏸ Monitor-Only</button>
+  </form>
+</div>
+{{end}}
+
 </body>
 </html>`