feat: add Mattermost session auth for browser requests

- Add dual auth path in ServeHTTP: shared secret (daemon) OR Mattermost session (browser)
- Read-only endpoints (GET /sessions, GET /health) accept either auth method
- Write endpoints (POST, PUT, DELETE) still require shared secret
- Browser requests authenticated via Mattermost-User-Id header (auto-injected by MM server)
- Unauthenticated requests now properly rejected with 401

Fixes: Issue #5 Phase 1 - RHS Panel auth fix

plugin/server/api.go

@@ -16,19 +16,40 @@ import (
 func (p *Plugin) ServeHTTP(c *plugin.Context, w http.ResponseWriter, r *http.Request) {
 	path := r.URL.Path
 
-	// Auth middleware: validate shared secret for write operations.
-	// Read-only endpoints (GET /sessions, GET /health) are accessible to any
-	// authenticated Mattermost user — no shared secret required.
+	// Auth middleware: two auth paths.
+	// 1. Shared secret (Bearer token) — used by the daemon for write operations.
+	// 2. Mattermost session (Mattermost-User-Id header) — used by browser requests.
+	//    The Mattermost server automatically injects this header for authenticated
+	//    requests routed through the plugin HTTP handler.
+	//
+	// Read-only endpoints (GET /sessions, GET /health) accept either auth method.
+	// Write endpoints (POST, PUT, DELETE) require the shared secret.
 	isReadOnly := r.Method == http.MethodGet && (path == "/api/v1/sessions" || path == "/api/v1/health")
-	if !isReadOnly {
-		config := p.getConfiguration()
-		if config.SharedSecret != "" {
-			auth := r.Header.Get("Authorization")
-			expected := "Bearer " + config.SharedSecret
-			if auth != expected {
-				http.Error(w, `{"error": "unauthorized"}`, http.StatusUnauthorized)
-				return
-			}
+
+	config := p.getConfiguration()
+	hasSharedSecret := false
+	if config.SharedSecret != "" {
+		auth := r.Header.Get("Authorization")
+		expected := "Bearer " + config.SharedSecret
+		hasSharedSecret = (auth == expected)
+	}
+
+	// Check Mattermost session auth (browser requests).
+	// The MM server injects Mattermost-User-Id for authenticated users.
+	mmUserID := r.Header.Get("Mattermost-User-Id")
+	hasMattermostSession := mmUserID != ""
+
+	if isReadOnly {
+		// Read-only: accept either shared secret OR Mattermost session
+		if !hasSharedSecret && !hasMattermostSession {
+			http.Error(w, `{"error": "unauthorized: valid Mattermost session or shared secret required"}`, http.StatusUnauthorized)
+			return
+		}
+	} else {
+		// Write operations: require shared secret (daemon auth)
+		if !hasSharedSecret {
+			http.Error(w, `{"error": "unauthorized"}`, http.StatusUnauthorized)
+			return
 		}
 	}