fix(webapp): route the graced flag writes through the traced transaction helper

Both graced writes called client.$transaction directly. The repo rule is to
use the $transaction helper from db.server, which adds the OTEL span and logs
the infrastructure errors the raw client swallows. One of these writes stamps
a cutover window and the other deletes flags, so a transaction that silently
did not run is the case most worth seeing.

The helper resolves undefined instead of throwing when it swallows such an
error, so both call sites now treat that as a failure the caller sees.

apps/webapp/app/v3/featureFlags.server.ts

@@ -1,6 +1,6 @@
 import { type z } from "zod";
 import type { PrismaClient } from "@trigger.dev/database";
-import { prisma, type PrismaClientOrTransaction } from "~/db.server";
+import { $transaction, prisma, type PrismaClientOrTransaction } from "~/db.server";
 import {
   FEATURE_FLAG,
   type FeatureFlagCatalogSchema,
@@ -271,11 +271,18 @@ export async function applyGlobalGracedFlips(
   requestedFlags: Partial<z.infer<typeof FeatureFlagCatalogSchema>>,
   graceMs: number
 ): Promise<{ key: string; value: any }[]> {
-  return client.$transaction(async (tx) => {
+  const applied = await $transaction(client, "applyGlobalGracedFlips", async (tx) => {
     await lockGracedGroups(tx);
     const stamped = await stampGracedGroups(tx, withoutDerivedKeys(requestedFlags), graceMs);
     return makeSetMultipleFlags(tx)(stamped as Partial<z.infer<typeof FeatureFlagCatalogSchema>>);
   });
+
+  // The helper resolves undefined rather than throwing when Prisma swallows an infrastructure
+  // error. This write stamps a cutover window, so a transaction that did not run must be loud.
+  if (!applied) {
+    throw new Error("applyGlobalGracedFlips: transaction did not complete");
+  }
+  return applied;
 }
 
 // Replace-semantics write for the global admin flags page: submitted flags upsert, omitted ones
@@ -298,7 +305,7 @@ export async function replaceGlobalFeatureFlags(
 ): Promise<void> {
   const requestedFlags = withoutDerivedKeys(params.requestedFlags);
 
-  await client.$transaction(async (tx) => {
+  const applied = await $transaction(client, "replaceGlobalFeatureFlags", async (tx) => {
     await lockGracedGroups(tx);
     const stamped = await stampGracedGroups(tx, requestedFlags, params.graceMs);
 
@@ -331,5 +338,12 @@ export async function replaceGlobalFeatureFlags(
     if (keysToDelete.length > 0) {
       await tx.featureFlag.deleteMany({ where: { key: { in: boundedIn(keysToDelete) } } });
     }
+
+    return true;
   });
+
+  // This write deletes flags, so a transaction that did not run must reach the caller.
+  if (!applied) {
+    throw new Error("replaceGlobalFeatureFlags: transaction did not complete");
+  }
 }