fix: dedup env var keys, add IDOR test, enforce body size limit

- Server-side duplicate key dedup (last wins) via deduplicateEnvPairs helper
- Cross-app isolation test verifying env var save scopes by app_id
- http.MaxBytesReader wraps request body with 1MB limit
- Test for oversized body rejection (400)

internal/handlers/handlers_test.go

@@ -664,6 +664,148 @@ func TestHandleEnvVarSaveEmptyKeyRejected(t *testing.T) {
 	assert.Equal(t, http.StatusBadRequest, recorder.Code)
 }
 
+// TestHandleEnvVarSaveDuplicateKeyDedup verifies that when the client
+// sends duplicate keys, the server deduplicates them (last wins).
+func TestHandleEnvVarSaveDuplicateKeyDedup(t *testing.T) {
+	t.Parallel()
+
+	testCtx := setupTestHandlers(t)
+	createdApp := createTestApp(t, testCtx, "envvar-dedup-app")
+
+	// Send two entries with the same key — last should win
+	body := `[{"key":"FOO","value":"first"},{"key":"BAR","value":"bar"},{"key":"FOO","value":"second"}]`
+
+	r := chi.NewRouter()
+	r.Post("/apps/{id}/env", testCtx.handlers.HandleEnvVarSave())
+
+	request := httptest.NewRequest(
+		http.MethodPost,
+		"/apps/"+createdApp.ID+"/env",
+		strings.NewReader(body),
+	)
+	request.Header.Set("Content-Type", "application/json")
+
+	recorder := httptest.NewRecorder()
+	r.ServeHTTP(recorder, request)
+
+	assert.Equal(t, http.StatusOK, recorder.Code)
+
+	envVars, err := models.FindEnvVarsByAppID(
+		context.Background(), testCtx.database, createdApp.ID,
+	)
+	require.NoError(t, err)
+	assert.Len(t, envVars, 2, "duplicate key should be deduplicated")
+
+	keys := make(map[string]string)
+	for _, ev := range envVars {
+		keys[ev.Key] = ev.Value
+	}
+
+	assert.Equal(t, "second", keys["FOO"], "last occurrence should win")
+	assert.Equal(t, "bar", keys["BAR"])
+}
+
+// TestHandleEnvVarSaveCrossAppIsolation verifies that posting env vars
+// to appA's endpoint does not affect appB's env vars (IDOR prevention).
+func TestHandleEnvVarSaveCrossAppIsolation(t *testing.T) {
+	t.Parallel()
+
+	testCtx := setupTestHandlers(t)
+	appA := createTestApp(t, testCtx, "envvar-iso-appA")
+	appB := createTestApp(t, testCtx, "envvar-iso-appB")
+
+	// Give appB some env vars
+	for _, kv := range [][2]string{{"B_KEY1", "b_val1"}, {"B_KEY2", "b_val2"}} {
+		ev := models.NewEnvVar(testCtx.database)
+		ev.AppID = appB.ID
+		ev.Key = kv[0]
+		ev.Value = kv[1]
+		require.NoError(t, ev.Save(context.Background()))
+	}
+
+	// POST new env vars to appA's endpoint
+	body := `[{"key":"A_KEY","value":"a_val"}]`
+
+	r := chi.NewRouter()
+	r.Post("/apps/{id}/env", testCtx.handlers.HandleEnvVarSave())
+
+	request := httptest.NewRequest(
+		http.MethodPost,
+		"/apps/"+appA.ID+"/env",
+		strings.NewReader(body),
+	)
+	request.Header.Set("Content-Type", "application/json")
+
+	recorder := httptest.NewRecorder()
+	r.ServeHTTP(recorder, request)
+
+	assert.Equal(t, http.StatusOK, recorder.Code)
+
+	// Verify appA has exactly what we sent
+	appAVars, err := models.FindEnvVarsByAppID(
+		context.Background(), testCtx.database, appA.ID,
+	)
+	require.NoError(t, err)
+	assert.Len(t, appAVars, 1)
+	assert.Equal(t, "A_KEY", appAVars[0].Key)
+
+	// Verify appB's env vars are completely untouched
+	appBVars, err := models.FindEnvVarsByAppID(
+		context.Background(), testCtx.database, appB.ID,
+	)
+	require.NoError(t, err)
+	assert.Len(t, appBVars, 2, "appB env vars must not be affected")
+
+	bKeys := make(map[string]string)
+	for _, ev := range appBVars {
+		bKeys[ev.Key] = ev.Value
+	}
+
+	assert.Equal(t, "b_val1", bKeys["B_KEY1"])
+	assert.Equal(t, "b_val2", bKeys["B_KEY2"])
+}
+
+// TestHandleEnvVarSaveBodySizeLimit verifies that a request body
+// exceeding the 1 MB limit is rejected.
+func TestHandleEnvVarSaveBodySizeLimit(t *testing.T) {
+	t.Parallel()
+
+	testCtx := setupTestHandlers(t)
+	createdApp := createTestApp(t, testCtx, "envvar-sizelimit-app")
+
+	// Build a JSON body that exceeds 1 MB
+	// Each entry is ~30 bytes; 40000 entries ≈ 1.2 MB
+	var sb strings.Builder
+
+	sb.WriteString("[")
+
+	for i := range 40000 {
+		if i > 0 {
+			sb.WriteString(",")
+		}
+
+		sb.WriteString(`{"key":"K` + strconv.Itoa(i) + `","value":"val"}`)
+	}
+
+	sb.WriteString("]")
+
+	r := chi.NewRouter()
+	r.Post("/apps/{id}/env", testCtx.handlers.HandleEnvVarSave())
+
+	request := httptest.NewRequest(
+		http.MethodPost,
+		"/apps/"+createdApp.ID+"/env",
+		strings.NewReader(sb.String()),
+	)
+	request.Header.Set("Content-Type", "application/json")
+
+	recorder := httptest.NewRecorder()
+	r.ServeHTTP(recorder, request)
+
+	assert.Equal(t, http.StatusBadRequest, recorder.Code,
+		"oversized body should be rejected with 400")
+}
+
 // TestDeleteLabelOwnershipVerification tests that deleting a label
 // via another app's URL path returns 404 (IDOR prevention).
 func TestDeleteLabelOwnershipVerification(t *testing.T) {