Keep what was typed when a target or webhook edit is refused (closes #381)

A refused save on the target edit page now shows the edit form again,
with the reason above it and every value submitted, instead of a bare
text page; the status codes are unchanged. The webhook edit page keeps
the submitted name, description and retention the same way, while the
page still reports the stored retention.

Target edits are validated by setTargetFromForm, which newTarget now
uses too, so the add and edit forms accept and refuse the same things.
An empty max_retries keeps the target's own count.

The browser test also saves both edit pages with refused values.

Model: opus-5-5

internal/handlers/source_management_test.go

@@ -509,6 +509,51 @@ func TestHandleSourceEditSubmit_InvalidRetentionIsRejected(
 	)
 }
 
+// TestHandleSourceEditSubmit_RefusedFormComesBack refuses an edit for
+// each reason the form can give and checks that the form comes back
+// with the reason and the name, description and retention submitted,
+// that the page still reports the stored retention, and that nothing
+// is saved.
+func TestHandleSourceEditSubmit_RefusedFormComesBack(t *testing.T) {
+	t.Parallel()
+
+	env := setupSourceTest(t)
+
+	refused := func(name, retention, reason string) {
+		t.Helper()
+
+		wh := seedWebhookWithRetention(t, env.db, 30)
+
+		submitted := wh
+		submitted.Name = name
+		submitted.Description = "a description worth keeping"
+
+		w := submitEdit(t, env, submitted, retention)
+		assert.Equal(t, http.StatusBadRequest, w.Code)
+
+		page := w.Body.String()
+		assert.Contains(t, page, `class="alert-error">`+reason)
+		assert.Contains(t, page, `name="name" value="`+name+`"`)
+		assert.Contains(t, page, ">a description worth keeping</textarea>")
+		assert.Contains(
+			t, page, `name="retention_days" value="`+retention+`"`,
+		)
+		assert.Contains(t, page, "Currently 30 days.")
+
+		var stored database.Webhook
+
+		require.NoError(
+			t, env.db.DB().First(&stored, "id = ?", wh.ID).Error,
+		)
+		assert.Equal(t, wh.Name, stored.Name)
+		assert.Empty(t, stored.Description)
+		assert.Equal(t, 30, stored.RetentionDays)
+	}
+
+	refused("", "45", "Name is required")
+	refused("kept-name", "nonsense", "Retention must be")
+}
+
 func TestHandleSourceEditSubmit_EmptyRetentionLeavesValueUnchanged(
 	t *testing.T,
 ) {
@@ -809,6 +854,10 @@ func TestHandleSourceEditSubmit_ArchiveNameTaken(t *testing.T) {
 	w := submitEdit(t, env, wh, "")
 	require.Equal(t, http.StatusConflict, w.Code)
 	assert.Contains(t, w.Body.String(), "archive-taken.db")
+	assert.Contains(
+		t, w.Body.String(), `name="name" value="`+renamedWebhookName+`"`,
+		"the form comes back with the name submitted",
+	)
 
 	var stored database.Webhook