fix(overpass): retry a joined caller's admission on its own quota

A caller that joins a cold reservation no longer inherits another client's
RATE_LIMITED result. Once the rejected reservation is released it retries:
it either owns the next reservation and is admitted on its own quota, or joins
an admitted fetch. Adds a two-client regression (exhausted owner, available
joiner, exhausted joiner coalescing onto the admitted fetch).

Also drops the duplicate memory publication for a disk hit in the handler;
preflight already sets it, the handler only trims.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

server/providers/overpass.js

@@ -204,10 +204,8 @@ function overpassProxy({ routing = {} } = {}) {
             return;
           }
         }
-        if (preflight.source === 'DISK') {
-          _overpassCache.set(cacheKey, preflight.payload);
-          trimOverpassCache();
-        }
+        // Preflight already published the disk hit to memory; only trim.
+        if (preflight.source === 'DISK') trimOverpassCache();
         sendOverpassResponse(
           res,
           preflight.payload,