Add mutex protection for config watcher reloads (re #748)

Introduced sync.RWMutex to protect concurrent access to configuration
fields (AuthUser, AuthPass, APITokens) that are modified by the
ConfigWatcher at runtime.

- Added global config.Mu RWMutex in internal/config/config.go
- Protected config updates in ConfigWatcher.reloadConfig() and reloadAPITokens()
- Protected config reads in CheckAuth and all API token handlers
- Protected Router.SetConfig() during full config reloads

This prevents race conditions when .env file changes trigger config
reloads while authentication handlers are reading the same fields.

internal/api/auth.go

@@ -196,6 +196,9 @@ func min(a, b int) int {
 
 // CheckAuth checks both basic auth and API token
 func CheckAuth(cfg *config.Config, w http.ResponseWriter, r *http.Request) bool {
+	config.Mu.RLock()
+	defer config.Mu.RUnlock()
+
 	// Check proxy auth first if configured
 	if cfg.ProxyAuthSecret != "" {
 		if valid, username, _ := CheckProxyAuth(cfg, r); valid {

internal/api/router.go

@@ -1201,6 +1201,9 @@ func (r *Router) SetConfig(cfg *config.Config) {
 		return
 	}
 
+	config.Mu.Lock()
+	defer config.Mu.Unlock()
+
 	if r.config == nil {
 		r.config = cfg
 	} else {
@@ -2839,6 +2842,9 @@ func (r *Router) handleConfig(w http.ResponseWriter, req *http.Request) {
 		return
 	}
 
+	config.Mu.RLock()
+	defer config.Mu.RUnlock()
+
 	// Return public configuration
 	config := map[string]interface{}{
 		"csrfProtection":    false, // Not implemented yet

internal/api/security_setup_fix.go

@@ -275,11 +275,13 @@ func handleQuickSecuritySetupFixed(r *Router) http.HandlerFunc {
 		}
 
 		// Update runtime config immediately with hashed token - no restart needed!
+		config.Mu.Lock()
 		r.config.AuthUser = setupRequest.Username
 		r.config.AuthPass = hashedPassword
 		r.config.APITokens = []config.APITokenRecord{*tokenRecord}
 		r.config.SortAPITokens()
 		r.config.APITokenEnabled = true
+		config.Mu.Unlock()
 
 		if r.persistence != nil {
 			if err := r.persistence.SaveAPITokens(r.config.APITokens); err != nil {
@@ -539,9 +541,11 @@ func (r *Router) HandleRegenerateAPIToken(w http.ResponseWriter, rq *http.Reques
 		return
 	}
 
+	config.Mu.Lock()
 	r.config.APITokens = []config.APITokenRecord{*tokenRecord}
 	r.config.SortAPITokens()
 	r.config.APITokenEnabled = true
+	config.Mu.Unlock()
 	log.Info().Msg("Runtime config updated with new API token - active immediately")
 
 	if r.persistence != nil {
@@ -672,7 +676,9 @@ func (r *Router) HandleValidateAPIToken(w http.ResponseWriter, rq *http.Request)
 	}
 
 	// Validate the token (compare hash)
+	config.Mu.RLock()
 	_, isValid := r.config.ValidateAPIToken(validateRequest.Token)
+	config.Mu.RUnlock()
 
 	// Log validation attempt without logging the token itself
 	if isValid {

internal/api/security_tokens.go

@@ -91,6 +91,9 @@ func (r *Router) handleListAPITokens(w http.ResponseWriter, req *http.Request) {
 		return
 	}
 
+	config.Mu.RLock()
+	defer config.Mu.RUnlock()
+
 	tokens := make([]apiTokenDTO, 0, len(r.config.APITokens))
 	for _, record := range r.config.APITokens {
 		tokens = append(tokens, toAPITokenDTO(record))
@@ -147,6 +150,9 @@ func (r *Router) handleCreateAPIToken(w http.ResponseWriter, req *http.Request)
 		return
 	}
 
+	config.Mu.Lock()
+	defer config.Mu.Unlock()
+
 	r.config.APITokens = append(r.config.APITokens, *record)
 	r.config.SortAPITokens()
 	r.config.APITokenEnabled = true
@@ -181,6 +187,9 @@ func (r *Router) handleDeleteAPIToken(w http.ResponseWriter, req *http.Request)
 		return
 	}
 
+	config.Mu.Lock()
+	defer config.Mu.Unlock()
+
 	removed := r.config.RemoveAPIToken(id)
 	if !removed {
 		http.Error(w, "Token not found", http.StatusNotFound)

internal/config/config.go

@@ -20,6 +20,8 @@ import (
 	"strings"
 	"time"
 
+	"sync"
+
 	"github.com/google/uuid"
 	"github.com/joho/godotenv"
 	"github.com/rcourtman/pulse-go-rewrite/internal/auth"
@@ -29,6 +31,10 @@ import (
 	"github.com/rs/zerolog/log"
 )
 
+// Mu protects concurrent access to the configuration,
+// particularly for fields updated by the config watcher (AuthUser, AuthPass, APITokens).
+var Mu sync.RWMutex
+
 const (
 	DefaultGuestMetadataMinRefresh    = 2 * time.Minute
 	DefaultGuestMetadataRefreshJitter = 45 * time.Second

internal/config/watcher.go

@@ -310,6 +310,9 @@ func (cw *ConfigWatcher) reloadConfig() {
 	}
 
 	// Apply auth user
+	Mu.Lock()
+	defer Mu.Unlock()
+
 	newUser := strings.Trim(envMap["PULSE_AUTH_USER"], "'\"")
 	if newUser != oldAuthUser {
 		cw.config.AuthUser = newUser
@@ -485,6 +488,8 @@ func (cw *ConfigWatcher) reloadAPITokens() {
 	}
 
 	// Only update if we successfully loaded tokens
+	Mu.Lock()
+	defer Mu.Unlock()
 	cw.config.APITokens = tokens
 	cw.config.SortAPITokens()
 	cw.config.APITokenEnabled = len(tokens) > 0