feat(backend): add TfL disruptions API with Gemini AI shortening

- Add TfL Unified API integration for real-time transport disruptions
- Implement batch AI shortening using Gemini 2.5 Flash-Lite
- Add in-memory caching with 1-hour TTL
- Support Tube, Overground, DLR, Elizabeth Line, and Tram
- Sort disruptions by severity with regex-based line name cleanup

Co-authored-by: Ona <no-reply@ona.com>

apps/backend/src/tfl/cache.ts

@@ -0,0 +1,57 @@
+/**
+ * Simple in-memory cache for shortened disruption descriptions
+ */
+
+interface CacheEntry {
+	shortened: string
+	timestamp: number
+}
+
+const cache = new Map<string, CacheEntry>()
+
+// Cache for 1 hour
+const CACHE_DURATION = 60 * 60 * 1000
+
+/**
+ * Get cached shortened description
+ */
+export function getCachedShortened(originalReason: string): string | null {
+	const entry = cache.get(originalReason)
+	
+	if (!entry) {
+		return null
+	}
+	
+	// Check if expired
+	if (Date.now() - entry.timestamp > CACHE_DURATION) {
+		cache.delete(originalReason)
+		return null
+	}
+	
+	return entry.shortened
+}
+
+/**
+ * Cache a shortened description
+ */
+export function setCachedShortened(originalReason: string, shortened: string): void {
+	cache.set(originalReason, {
+		shortened,
+		timestamp: Date.now(),
+	})
+}
+
+/**
+ * Clear expired cache entries
+ */
+export function clearExpiredCache(): void {
+	const now = Date.now()
+	for (const [key, entry] of cache.entries()) {
+		if (now - entry.timestamp > CACHE_DURATION) {
+			cache.delete(key)
+		}
+	}
+}
+
+// Clear expired entries every 10 minutes
+setInterval(clearExpiredCache, 10 * 60 * 1000)