feat: wire GITEA_TOKEN env var as auth fallback for single-user deployments

Update Auth middleware to accept a fallbackToken parameter. When no
per-user cookie token is present and GITEA_TOKEN is set in the
environment, the middleware uses the env token instead of redirecting
to /settings. Cookie tokens still take precedence over the fallback.

Add three new unit tests covering: fallback used when no cookie,
cookie takes precedence over fallback, and redirect when neither is set.

Closes leeworks-agents/gitea-mobile#125

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/middleware/auth.go

@@ -23,9 +23,12 @@ func TokenFromContext(ctx context.Context) string {
 }
 
 // Auth returns middleware that checks for a valid token cookie.
+// If no cookie token is found and fallbackToken is non-empty, the fallback
+// token is used instead (useful for single-user or service-account deployments
+// where GITEA_TOKEN is set in the environment).
 // Unauthenticated requests are redirected to the settings page.
 // The /health, /settings, and /static/ paths are exempt from auth.
-func Auth(sessionSecret string) func(http.Handler) http.Handler {
+func Auth(sessionSecret, fallbackToken string) func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			// Skip auth for exempt paths.
@@ -37,6 +40,13 @@ func Auth(sessionSecret string) func(http.Handler) http.Handler {
 
 			token, err := auth.GetToken(r, sessionSecret)
 			if err != nil || token == "" {
+				// Fall back to environment token if available.
+				if fallbackToken != "" {
+					slog.Debug("using fallback token from environment", "path", path)
+					ctx := context.WithValue(r.Context(), TokenContextKey, fallbackToken)
+					next.ServeHTTP(w, r.WithContext(ctx))
+					return
+				}
 				slog.Debug("unauthenticated request, redirecting to settings", "path", path, "error", err)
 				http.Redirect(w, r, "/settings", http.StatusSeeOther)
 				return