fix: don't hide async exceptions in logs

Fixes #4646. Using the repro on #4646, this now produces the log:

```
11/Feb/2026:09:40:08 -0500: Warp server error: stack overflow
```

When:
```
$ curl localhost:3000/
curl: (52) Empty reply from server
```

(cherry picked from commit e95e815483)

src/PostgREST/Observation.hs

@@ -63,6 +63,7 @@ data Observation
   | PoolRequestFullfilled
   | JwtCacheLookup Bool
   | JwtCacheEviction
+  | WarpErrorObs Text
 
 data ObsFatalError = ServerAuthError | ServerPgrstBug | ServerError42P05 | ServerError08P01
 
@@ -157,6 +158,8 @@ observationMessage = \case
     "Looked up a JWT in JWT cache"
   JwtCacheEviction ->
     "Evicted entry from JWT cache"
+  WarpErrorObs txt ->
+    "Warp server error: " <> txt
   where
     showMillis :: Double -> Text
     showMillis x = toS $ showFFloat (Just 1) x ""