fix(webapp): narrow run-detail buffer fallback to RunNotInPgError

The loader's catch block was treating every RunPresenter error as a
PG miss and trying the mollifier buffer fallback. That meant a
downstream trace-query or event-repository failure after the PG row
was found would either return stale synth data (if a buffer entry
happened to still be present during the drainer materialisation race
window) or silently mask the real error.

Narrow the fallback to `RunNotInPgError` — the typed signal
RunPresenter throws specifically for "this run is not in PG and may
be in the buffer" — and re-throw all other errors. Matches the
design intent stated on the class itself.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: d-cs

apps/webapp/app/routes/_app.orgs.$organizationSlug.projects.$projectParam.env.$envParam.runs.$runParam/route.tsx

@@ -92,7 +92,11 @@ import { env } from "~/env.server";
 import { findProjectBySlug } from "~/models/project.server";
 import { findEnvironmentBySlug } from "~/models/runtimeEnvironment.server";
 import { NextRunListPresenter } from "~/presenters/v3/NextRunListPresenter.server";
-import { RunEnvironmentMismatchError, RunPresenter } from "~/presenters/v3/RunPresenter.server";
+import {
+  RunEnvironmentMismatchError,
+  RunNotInPgError,
+  RunPresenter,
+} from "~/presenters/v3/RunPresenter.server";
 import { findRunByIdWithMollifierFallback } from "~/v3/mollifier/readFallback.server";
 import { buildSyntheticRunHeader } from "~/v3/mollifier/syntheticRunHeader.server";
 import { buildSyntheticTraceForBufferedRun } from "~/v3/mollifier/syntheticTrace.server";
@@ -281,6 +285,18 @@ export const loader = async ({ request, params }: LoaderFunctionArgs) => {
       );
     }
 
+    // Only fall back to the mollifier buffer on a genuine PG miss. Any
+    // other error (DB timeout during trace queries, event-repository
+    // failure, etc.) means the run WAS in PG but a downstream lookup
+    // failed — falling back to the buffer here would either return a
+    // stale synth entry if one happens to exist in the brief drainer-
+    // materialisation race window, or quietly mask the real failure.
+    // `RunNotInPgError` is the typed signal RunPresenter throws for the
+    // route loader's specific case (`RunPresenter.server.ts:130`).
+    if (!(error instanceof RunNotInPgError)) {
+      throw error;
+    }
+
     // PG miss → try the mollifier buffer. When the gate diverts a trigger
     // the run sits in Redis until the drainer materialises it; without
     // this fallback the run-detail page 404s for the brief buffered window