fix(cache): invalidate public profile cache after platform link mutations

Root cause: createPlatformLink, updatePlatformLink, deletePlatformLink, and
reorderLinks all mutated the database but never called redis.del on the
profile:<username> cache key, leaving stale data served to viewers until
the 5-minute TTL expired naturally.

Fix: Add a private invalidateProfileCacheForUser helper that resolves the
username via a lightweight SELECT then calls redis.del. All four mutation
functions now await this helper after a successful DB write so the cache
is cleared immediately. Cache invalidation is skipped when Redis is absent
and errors are caught and logged non-fatally so a Redis blip never fails
a mutation request.

Also fix the DELETE /api/cards/:id route handler which checked error codes
as return values; the service throws errors, so the handler now catches them.
Fix cards.test.ts duplicate buildApp declaration, and apply the PlatformLink
type fix to cardService.ts (upstream/main has not yet merged that PR).

Tests: 21 new tests in profile-cache.test.ts cover cache hit/miss lifecycle,
all four mutation paths, failed mutations, non-existent links, Redis-absent
mode, consecutive mutations, cache repopulation, and non-fatal Redis errors.

Author: Ridanshi

apps/backend/src/__tests__/cards.test.ts

@@ -1,4 +1,4 @@
-import Fastify, { type FastifyInstance, type FastifyRequest } from 'fastify';
+import Fastify, { type FastifyInstance } from 'fastify';
 import { describe, it, expect, beforeEach, vi } from 'vitest';
 
 import { cardRoutes } from '../routes/cards.js';
@@ -53,10 +53,6 @@ function wireTransaction(): void {
 }
 
 async function buildApp(): Promise<FastifyInstance> {
-  const app = Fastify({ logger: false });
-  app.decorate('prisma', mockPrisma);
-  app.decorate('authenticate', async (request: FastifyRequest & { user?: { id: string } }) => {
-async function buildApp():Promise<FastifyInstance> {
   const app = Fastify({ logger: false });
   app.decorate('prisma', mockPrisma as unknown as PrismaClient);
   app.decorate('authenticate', async (request: any) => {