test: serialize external db sync tests via forks pool

Author: BilalG1

apps/e2e/vitest.config.ts

@@ -14,6 +14,19 @@ export default mergeConfig(
         "./tests/setup.ts",
       ],
       snapshotSerializers: ["./tests/snapshot-serializer.ts"],
+      poolMatchGlobs: [
+        ['**/tests/backend/endpoints/api/v1/external-db-sync*.test.ts', 'forks'],
+      ],
+      poolOptions: {
+        forks: {
+          maxForks: 1,
+          minForks: 1,
+        },
+        threads: {
+          maxThreads: 8,
+          minThreads: 1,
+        },
+      },
     },
   }),
 )

claude/CLAUDE-KNOWLEDGE.md

@@ -11,3 +11,6 @@ A: The generated `.next/types/validator.ts` can keep stale imports for removed r
 
 Q: Why can external DB sync tests time out in dev-focused GitHub workflows?
 A: The first calls to `/api/latest/internal/external-db-sync/sequencer` and `/poller` can be slow in dev mode and hit Undici's headers timeout; prewarming those endpoints with the cron secret, retrying header-timeout failures in the test helper, or running tests single-worker are viable mitigations.
+
+Q: How can we serialize only the external DB sync Vitest files while keeping the rest parallel?
+A: Use `poolMatchGlobs` to route the external DB sync test globs to the `forks` pool and set `poolOptions.forks.{minForks,maxForks}=1` in `apps/e2e/vitest.config.ts`; keep the default threads pool for all other tests.