fix: CachedImage Animated.Value, SWR cache invalidation, background task timeout (#620 #621 #622)

[BashMan11]

Summary

Fixes three bugs assigned to me in one atomic commit.

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Closes #621 — CachedImage Animated.Value in render body

File: src/components/ui/CachedImage.tsx

• Imported Animated from react-native
• Changed opacity to const opacity = useRef(new Animated.Value(0)).current — reference is now stable across re-renders
• Removed expo-image's built-in transition={250} and replaced with an explicit Animated.timing fade-in that fires once on onLoadingComplete
• Wrapped ExpoImage in Animated.View driven by the stable opacity ref

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Closes #620 — SWR cache does not invalidate after POST/PUT mutations

Files: src/services/api/cache.ts, src/config/apiCacheConfig.ts, src/services/api/axios.config.ts

• Added invalidateByPattern(pattern: RegExp) to cache.ts — iterates memory store and persistent layer
• Created src/config/apiCacheConfig.ts with MUTATION_INVALIDATION_MAP mapping URL patterns + HTTP methods → invalidation RegExps (courses, users, lessons)
• Updated axios response interceptor to look up each successful mutation in the map and call invalidateByPattern synchronously before returning the response

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Closes #622 — backgroundTaskScheduler tasks unbounded (iOS 30-second limit)

Files: src/utils/backgroundTaskScheduler.ts, src/services/syncService.ts

• Added timeoutPromise(ms) helper and runWithTimeout(taskFn, taskName, timeoutMs=25000) that wraps execution in Promise.race
• On timeout: logs warn with task name and elapsed ms, returns { timedOut: true, taskDurationMs }
• enqueueLowPriorityTask now accepts an optional name parameter and routes through runWithTimeout
• Added SyncService.runBackgroundSync() — calls backgroundScheduler.runWithTimeout and checkpoints isSyncing=false on timeout so the next run can resume cleanly

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Tests

src/__tests__/issues/fixes_620_621_622.test.ts

Issue	Test
#621	Object.is confirms opacity ref is identical across 5 re-renders
#620	Cache miss immediately after invalidateByPattern call; count returned correctly
#622	Fake timers confirm timedOut=true fires at exactly 25 000 ms

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