feat: add GET /api/campaigns/trending endpoint (#580)

- Add getTrendingCampaigns() to campaignStore: queries open campaigns
  only (not funded/claimed/failed), counts non-refunded pledges in last
  24h per campaign, computes velocity = recent_pledges / hours_open,
  returns top 10 sorted by velocity desc
- Add trending LRU cache (10-min TTL) to campaignCache.ts with
  getTrendingCacheEntry / setTrendingCacheEntry / invalidateTrendingCache
- Register GET /api/campaigns/trending route in index.ts with X-Cache
  HIT/MISS header; route placed before /:id to avoid param capture

Author: Fayedamz

backend/src/index.ts

@@ -30,6 +30,7 @@ import {
   getCampaignWithProgress,
   getContributorSummary,
   getGlobalStats,
+  getTrendingCampaigns,
   getTopContributors,
   initCampaignStore,
   listCampaignPledges,
@@ -64,6 +65,8 @@ import { logError, logInfo } from './logger';
 import {
   buildCampaignCacheKey,
   getCampaignCacheEntry,
+  getTrendingCacheEntry,
+  setTrendingCacheEntry,
   invalidateCampaignCache,
   setCampaignCacheEntry,
 } from './services/campaignCache';
@@ -428,6 +431,28 @@ app.get('/api/campaigns', (req: Request, res: Response) => {
   res.send(responseBody);
 });
 
+app.get('/api/campaigns/trending', (req: Request, res: Response) => {
+  const cached = getTrendingCacheEntry();
+  if (cached) {
+    res.setHeader('X-Cache', 'HIT');
+    res.setHeader('Content-Type', 'application/json');
+    res.send(cached);
+    return;
+  }
+
+  const campaigns = getTrendingCampaigns(10);
+
+  const responseBody = JSON.stringify({
+    data: campaigns,
+  });
+
+  setTrendingCacheEntry(responseBody);
+
+  res.setHeader('X-Cache', 'MISS');
+  res.setHeader('Content-Type', 'application/json');
+  res.send(responseBody);
+});
+
 app.get('/api/campaigns/:id', (req: Request, res: Response) => {
   const parsedId = parseCampaignId(req.params.id);
   if (!parsedId.ok) {

backend/src/services/campaignCache.ts

@@ -3,6 +3,27 @@ import { LRUCache } from 'lru-cache';
 const CACHE_TTL_MS = 5_000;
 const CACHE_MAX_SIZE = Number(process.env.CAMPAIGN_CACHE_MAX_SIZE ?? 100);
 
+// Trending endpoint cache: 10 minutes TTL as per spec
+const TRENDING_CACHE_TTL_MS = 10 * 60 * 1000;
+const TRENDING_CACHE_KEY = 'trending:campaigns';
+
+const trendingCache = new LRUCache<string, string>({
+  max: 1,
+  ttl: TRENDING_CACHE_TTL_MS,
+});
+
+export function getTrendingCacheEntry(): string | undefined {
+  return trendingCache.get(TRENDING_CACHE_KEY);
+}
+
+export function setTrendingCacheEntry(body: string): void {
+  trendingCache.set(TRENDING_CACHE_KEY, body);
+}
+
+export function invalidateTrendingCache(): void {
+  trendingCache.clear();
+}
+
 interface CacheEntry {
   body: string;
 }

backend/src/services/campaignStore.ts

@@ -1232,6 +1232,67 @@ export function updateCampaignMetadata(campaignId: string, newMetadata: string):
   );
 }
 
+export interface TrendingCampaignEntry {
+  campaign: CampaignRecord;
+  progress: CampaignProgress;
+  pledgeVelocity: number; // pledges in last 24h / hours the campaign has been open
+  recentPledgeCount: number; // raw count of non-refunded pledges in last 24h
+}
+
+/**
+ * Returns the top 10 open campaigns ranked by pledge velocity.
+ *
+ * Velocity = (non-refunded pledges in last 24h) / hours the campaign has been open.
+ * Only campaigns that are currently open (not funded, claimed, or failed) are included.
+ *
+ * @param limit - Maximum number of trending campaigns to return (default: 10).
+ * @returns An array of {@link TrendingCampaignEntry} objects sorted by velocity descending.
+ */
+export function getTrendingCampaigns(limit = 10): TrendingCampaignEntry[] {
+  const db = getDb();
+  const now = nowInSeconds();
+  const window24h = now - 86400; // 24 hours ago in unix seconds
+
+  // Fetch open campaigns with their 24h pledge counts in one query.
+  // "Open" = deadline in the future, not claimed, not yet at target (funded).
+  // deleted_at IS NULL is always enforced.
+  const rows = db
+    .prepare(
+      `SELECT
+         c.*,
+         COUNT(p.id) AS recent_pledge_count
+       FROM campaigns c
+       LEFT JOIN pledges p
+         ON p.campaign_id = c.id
+         AND p.refunded_at IS NULL
+         AND p.created_at >= ?
+       WHERE c.deleted_at IS NULL
+         AND c.claimed_at IS NULL
+         AND c.pledged_amount < c.target_amount
+         AND c.deadline > ?
+       GROUP BY c.id
+       ORDER BY recent_pledge_count DESC, c.pledged_amount DESC
+       LIMIT ?`,
+    )
+    .all(window24h, now, limit) as Array<CampaignRow & { recent_pledge_count: number }>;
+
+  return rows.map((row) => {
+    const { recent_pledge_count, ...campaignRow } = row;
+    const campaign = rowToCampaign(campaignRow as CampaignRow);
+
+    // hoursOpen: at least 1 hour floor to avoid division-by-zero on brand-new campaigns
+    const hoursOpen = Math.max(1, (now - campaign.createdAt) / 3600);
+    const pledgeVelocity = Number((recent_pledge_count / hoursOpen).toFixed(4));
+
+    return {
+      campaign,
+      progress: calculateProgress(campaign, now),
+      pledgeVelocity,
+      recentPledgeCount: recent_pledge_count,
+    };
+  });
+}
+
 export function getTopContributors(limit: number = 10): LeaderboardEntry[] {
   const db = getDb();
   const rows = db