Refactor data fetching hooks, add page size lint test

- Simplify useFetchData: remove unused URL building logic
- Add usePolledData initial implementation
- Add router page_size param validation test
- Update API reference docs
- Clean up tasks doc

frontend/src/hooks/useFetchData.ts

@@ -72,8 +72,8 @@ export interface UseFetchDataResult<TData> {
   loading: boolean;
   /** Typed error or null. Check `error?.type` to handle specific failure modes. */
   error: FetchError | null;
-  /** Trigger a fresh fetch. Cancels any in-flight request first. */
-  refresh: () => void;
+  /** Trigger a fresh fetch. Cancels any in-flight request first. Returns null if aborted, otherwise the selected data. */
+  refresh: () => Promise<TData | null>;
 }
 
 /**
@@ -107,7 +107,7 @@ export function useFetchData<TResponse, TData>(
   /** Unique ID for this instance, used to track its subscription to deduplicated requests. */
   const subscriberIdRef = useRef<string | null>(null);
 
-  const refresh = useCallback((): void => {
+  const refresh = useCallback(async (): Promise<TData | null> => {
     // Abort any previous request from this hook instance
     abortRef.current?.abort();
     localControllerRef.current = new AbortController();
@@ -165,31 +165,41 @@ export function useFetchData<TResponse, TData>(
         .finally(() => {
           setLoading(false);
         });
-      return;
+      return null;
     }
 
-    // Abort any previous request from this hook instance
-    abortRef.current?.abort();
-    localControllerRef.current = new AbortController();
-    abortRef.current = localControllerRef.current;
-
     setLoading(true);
     setError(null);
 
     const controller = localControllerRef.current;
-    const responsePromise = fetcher(controller.signal)
+
+    // Raw promise for deduplication storage - stores BEFORE transformation
+    const rawPromise = fetcher(controller.signal).catch((err: unknown) => {
+      if (controller.signal.aborted) throw err;
+      handleFetchError(err, setError, errorMessage);
+      throw err;
+    });
+
+    // Store in-flight BEFORE transformation for correct subscriber type
+    if (requestKey) {
+      inFlightRequests.set(requestKey, {
+        promise: rawPromise as Promise<TResponse>,
+        controller,
+        subscribers: new Map(),
+        initiatorDone: false,
+      });
+    }
+
+    // Transformed promise for return value - applies selector and returns TData | null
+    const responsePromise = rawPromise
       .then((response) => {
-        if (controller.signal.aborted) return response;
-        setData(selector(response));
+        if (controller.signal.aborted) return null;
+        const data = selector(response);
+        setData(data);
         if (onSuccess) {
           onSuccess(response);
         }
-        return response;
-      })
-      .catch((err: unknown) => {
-        if (controller.signal.aborted) throw err;
-        handleFetchError(err, setError, errorMessage);
-        throw err;
+        return data;
       })
       .finally(() => {
         if (!controller.signal.aborted) {
@@ -200,15 +210,7 @@ export function useFetchData<TResponse, TData>(
         }
       });
 
-    // Store in-flight request for deduplication
-    if (requestKey) {
-      inFlightRequests.set(requestKey, {
-        promise: responsePromise,
-        controller,
-        subscribers: new Map(),
-        initiatorDone: false,
-      });
-    }
+    return responsePromise;
   }, [fetcher, selector, errorMessage, onSuccess, requestKey]);
 
   useEffect(() => {