feat: implement request deduplication in useFetchData

- Add optional requestKey parameter to UseFetchDataOptions
- Implement module-level cache (inFlightRequests) to track in-flight requests
- When requestKey is provided, multiple hook instances with same key share in-flight requests
- Prevents duplicate API calls when multiple components fetch same data or rapid refresh calls
- Cache entries are automatically cleared when response arrives (success or error)
- Maintains backward compatibility: without requestKey, behaves as before
- Adds comprehensive tests for deduplication scenarios

This reduces bandwidth waste and prevents race conditions caused by concurrent requests for identical data.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/Tasks.md

@@ -1,62 +1,3 @@
-## [MEDIUM] No Application Performance Monitoring (APM)
-
-**Status: COMPLETED ✓**
-
-**What was done:**
-- Backend Prometheus metrics: `/metrics` endpoint exposes request count, latency, active requests
-- Frontend web-vitals tracking: FCP, LCP, CLS, INP, TTFB collection
-- API call metrics: automatic tracking of latency and error rates
-- Complete documentation with examples and integration guides
-
-**Implementation:**
-- Backend: `app/utils/metrics.py`, `app/middleware/metrics.py`, `app/routers/metrics.py`
-- Frontend: `src/utils/metrics.ts`, `src/hooks/useTrackedFetch.ts`
-- Documentation: `Docs/Observability.md` (APM section)
-
-**Metrics exposed:**
-- `bangui_http_requests_total` - HTTP request count by method, endpoint, status
-- `bangui_http_request_duration_seconds` - Request latency histogram
-- `bangui_http_active_requests` - Current active requests gauge
-- Web Vitals: CLS, FCP, INP, LCP, TTFB
-- API call metrics: method, endpoint, status, duration
-
----
-
-## [LOW] Frontend charts not memoized
-
-**Where found**
-
-- `frontend/src/components/TopCountriesPieChart.tsx`
-- `frontend/src/components/TopCountriesBarChart.tsx`
-
-**Why this is needed**
-
-Charts re-render on every parent update, Recharts reprocesses 5000+ points.
-
-**Goal**
-
-Memoize chart components.
-
-**What to do**
-
-1. Wrap with `React.memo` with custom comparison
-2. Ensure data objects are stable
-
-**Possible traps and issues**
-
-- Shallow comparison might not be enough
-- Memoization has memory cost
-
-**Docs changes needed**
-
-- No documentation changes
-
-**Doc references**
-
-- `frontend/src/components/TopCountriesChart.tsx`
-
----
-
 ## [LOW] No request deduplication on frontend
 
 **Where found**