docs: verify API calls and performance

docs/api-performance.md

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+# API calls and performance
+
+## How we tested
+
+We opened the deployed application at http://it2810-23.idi.ntnu.no/project1/
+and inspected the Network tab in the browser's developer tools, then confirmed
+the findings by tracking every request via the Resource Timing API
+(`performance.getEntriesByType('resource')`), which records every request made
+since the page loaded regardless of when we started watching.
+
+We checked requests to the REST API (`countries.dev`) at each stage:
+
+- On application start.
+- While filtering by region, including switching back to "All regions".
+- While changing the sort order.
+- While navigating between countries with Previous/Next and the country
+  selector.
+- After leaving the page idle for 10 seconds, to rule out polling or a
+  refetch loop.
+
+## Findings
+
+The application makes exactly **one** request to the REST API per session:
+
+```
+GET https://countries.dev/region/europe
+```
+
+This happens once when the app starts. Region filtering and sorting are both
+done client-side on the already-fetched list — the "Region" dropdown filters
+by subregion in memory, and "Sort by" reorders the same array — so neither
+triggers a new request. Navigating between countries only changes which
+already-fetched country is displayed. No additional requests occurred during
+any of the above, and none occurred while idle.
+
+## TanStack Query configuration
+
+No changes were needed. The existing configuration in `src/lib/queryClient.ts`
+already supports this:
+
+- `staleTime: 5 * 60 * 1000` — fetched data is treated as fresh for 5 minutes,
+  so remounting the country list within that window doesn't refetch.
+- `refetchOnWindowFocus: false` — switching tabs or windows doesn't trigger a
+  refetch.
+- `retry: 1` — a failed request is retried once, not looped indefinitely.
+
+Since there is only one query in the app (`useCountries`, keyed by region) and
+the UI never changes the region it's called with, there is only ever one
+active query, and it is never invalidated during normal use.