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_test.go

@@ -11,7 +11,7 @@ import (
 const testSecret = "test-secret-that-is-at-least-32-chars-long"
 
 func TestAuth_HealthBypass(t *testing.T) {
-	handler := Auth(testSecret)(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	handler := Auth(testSecret, "")(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusOK)
 	}))
 
@@ -25,7 +25,7 @@ func TestAuth_HealthBypass(t *testing.T) {
 }
 
 func TestAuth_SettingsBypass(t *testing.T) {
-	handler := Auth(testSecret)(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	handler := Auth(testSecret, "")(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusOK)
 	}))
 
@@ -39,7 +39,7 @@ func TestAuth_SettingsBypass(t *testing.T) {
 }
 
 func TestAuth_RedirectWithoutToken(t *testing.T) {
-	handler := Auth(testSecret)(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	handler := Auth(testSecret, "")(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusOK)
 	}))
 
@@ -57,7 +57,7 @@ func TestAuth_RedirectWithoutToken(t *testing.T) {
 
 func TestAuth_PassWithToken(t *testing.T) {
 	called := false
-	handler := Auth(testSecret)(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	handler := Auth(testSecret, "")(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		called = true
 		token := TokenFromContext(r.Context())
 		if token != "my-token" {
@@ -83,3 +83,72 @@ func TestAuth_PassWithToken(t *testing.T) {
 		t.Errorf("status = %d, want %d", w.Code, http.StatusOK)
 	}
 }
+
+func TestAuth_FallbackToken_UsedWhenNoCookie(t *testing.T) {
+	called := false
+	handler := Auth(testSecret, "env-fallback-token")(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		called = true
+		token := TokenFromContext(r.Context())
+		if token != "env-fallback-token" {
+			t.Errorf("token = %q, want %q", token, "env-fallback-token")
+		}
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	req := httptest.NewRequest(http.MethodGet, "/", nil)
+	w := httptest.NewRecorder()
+	handler.ServeHTTP(w, req)
+
+	if !called {
+		t.Error("next handler was not called with fallback token")
+	}
+	if w.Code != http.StatusOK {
+		t.Errorf("status = %d, want %d", w.Code, http.StatusOK)
+	}
+}
+
+func TestAuth_FallbackToken_CookieTakesPrecedence(t *testing.T) {
+	called := false
+	handler := Auth(testSecret, "env-fallback-token")(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		called = true
+		token := TokenFromContext(r.Context())
+		if token != "cookie-token" {
+			t.Errorf("token = %q, want %q (cookie should take precedence over fallback)", token, "cookie-token")
+		}
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	// Set a cookie token.
+	cookieW := httptest.NewRecorder()
+	auth.SetTokenCookie(cookieW, "cookie-token", testSecret, false)
+	cookie := cookieW.Result().Cookies()[0]
+
+	req := httptest.NewRequest(http.MethodGet, "/", nil)
+	req.AddCookie(cookie)
+	w := httptest.NewRecorder()
+	handler.ServeHTTP(w, req)
+
+	if !called {
+		t.Error("next handler was not called")
+	}
+	if w.Code != http.StatusOK {
+		t.Errorf("status = %d, want %d", w.Code, http.StatusOK)
+	}
+}
+
+func TestAuth_NoFallbackToken_RedirectsWithoutCookie(t *testing.T) {
+	handler := Auth(testSecret, "")(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	req := httptest.NewRequest(http.MethodGet, "/issues", nil)
+	w := httptest.NewRecorder()
+	handler.ServeHTTP(w, req)
+
+	if w.Code != http.StatusSeeOther {
+		t.Errorf("status = %d, want %d", w.Code, http.StatusSeeOther)
+	}
+	if loc := w.Header().Get("Location"); loc != "/settings" {
+		t.Errorf("Location = %q, want %q", loc, "/settings")
+	}
+}