feat: add controller self-update mechanism (v0.16.0)

New selfupdate package: version parsing, audit state file, updater with
Gitea registry V2 check, docker pull + compose rewrite + compose up flow.

- API: /api/selfupdate/{status,check,update} with session+bearer auth
- UI: Settings "Verzió és frissítés" card with check/install buttons + JS polling
- Scheduler: periodic check (6h default) + optional daily auto-update
- Notifications: success/failure on post-update startup verification
- Alert: info banner when update available
- docker-compose.yml: add directory bind mount for compose file access

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

controller/internal/selfupdate/state.go

@@ -0,0 +1,71 @@
+package selfupdate
+
+import (
+	"encoding/json"
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+)
+
+const stateFileName = "update-state.json"
+
+// UpdateState tracks the last update attempt. Persisted to disk as audit log.
+type UpdateState struct {
+	Status          string `json:"status"`                   // "pending", "success", "failed"
+	PreviousVersion string `json:"previous_version"`
+	PreviousImage   string `json:"previous_image"`
+	TargetVersion   string `json:"target_version"`
+	TargetImage     string `json:"target_image"`
+	InitiatedAt     string `json:"initiated_at"`             // RFC3339
+	InitiatedBy     string `json:"initiated_by"`             // "manual" or "auto"
+	CompletedAt     string `json:"completed_at,omitempty"`
+	Error           string `json:"error,omitempty"`
+}
+
+// LoadState reads the update state file. Returns nil, nil if file doesn't exist.
+func LoadState(dataDir string) (*UpdateState, error) {
+	path := filepath.Join(dataDir, stateFileName)
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+		return nil, fmt.Errorf("reading state file: %w", err)
+	}
+
+	var state UpdateState
+	if err := json.Unmarshal(data, &state); err != nil {
+		return nil, fmt.Errorf("parsing state file: %w", err)
+	}
+	return &state, nil
+}
+
+// SaveState writes the state file atomically (write to .tmp, then rename).
+func SaveState(dataDir string, state *UpdateState) error {
+	path := filepath.Join(dataDir, stateFileName)
+	tmpPath := path + ".tmp"
+
+	data, err := json.MarshalIndent(state, "", "  ")
+	if err != nil {
+		return fmt.Errorf("marshaling state: %w", err)
+	}
+
+	if err := os.WriteFile(tmpPath, data, 0644); err != nil {
+		return fmt.Errorf("writing temp state file: %w", err)
+	}
+
+	if err := os.Rename(tmpPath, path); err != nil {
+		return fmt.Errorf("renaming state file: %w", err)
+	}
+
+	return nil
+}
+
+// ClearState removes the state file. Used for cleanup.
+func ClearState(dataDir string, logger *log.Logger) {
+	path := filepath.Join(dataDir, stateFileName)
+	if err := os.Remove(path); err != nil && !os.IsNotExist(err) {
+		logger.Printf("[WARN] Failed to clear update state file: %v", err)
+	}
+}