fix(brief): bundle resvg linux-x64-gnu native binding with carousel fn (#3204)

* fix(brief): bundle resvg linux-x64-gnu native binding with carousel fn

Real root cause of every Telegram carousel WEBPAGE_CURL_FAILED
since PR #3174 merged. Not middleware (last PR fixed that
theoretical path but not the observed failure). The Vercel
function itself crashes HTTP 500 FUNCTION_INVOCATION_FAILED on
every request including OPTIONS - the isolate can't initialise.

The handler imports brief-carousel-render which lazy-imports
@resvg/resvg-js. That package's js-binding.js does runtime
require(@resvg/resvg-js-<platform>-<arch>-<libc>). On Vercel
Lambda (Amazon Linux 2 glibc) that resolves to
@resvg/resvg-js-linux-x64-gnu. Vercel nft tracing does NOT
follow this conditional require so the optional peer package
isnt bundled. Cold start throws MODULE_NOT_FOUND, isolate
crashes, Vercel returns FUNCTION_INVOCATION_FAILED, Telegram
reports WEBPAGE_CURL_FAILED.

Fix: vercel.json functions.includeFiles forces linux-x64-gnu
binding into the carousel functions bundle. Only this route
needs it; every other api route is unaffected.

Verified:
- deploy-config tests 21/21 pass
- JSON valid
- Reproduced 500 via curl on all methods and UAs
- resvg-js/js-binding.js confirms linux-x64-gnu is the runtime
  binary on Amazon Linux 2 glibc

Post-merge: curl with TelegramBot UA should return 200 image/png
instead of 500; next cron tick should clear the Railway
[digest] Telegram carousel 400 line.

* Address Greptile P2s: regression guard + arch-assumption reasoning

Two P2 findings on PR #3204:

P2 #1 (inline on vercel.json:6): Platform architecture assumption
undocumented. If Vercel migrates to Graviton/arm64 Lambda the
cold-start crash silently returns. vercel.json is strict JSON so
comments aren't possible inline.

P2 #2 (tests/deploy-config.test.mjs:17): No regression guard for
the carousel includeFiles rule. A future vercel.json tidy-up
could silently revert the fix with no CI signal.

Fixed both in a single block:

- New describe() in deploy-config.test.mjs asserts the carousel
  route's functions entry exists AND its includeFiles points at
  @resvg/resvg-js-linux-x64-gnu. Any drift fails the build.
- The block comment above it documents the Amazon Linux 2 x86_64
  glibc assumption that would have lived next to the includeFiles
  entry if JSON supported comments. Includes the Graviton/arm64
  migration pointer.

tests 22/22 pass (was 21, +1 new).

vercel.json

@@ -1,6 +1,11 @@
 {
   "ignoreCommand": "bash scripts/vercel-ignore.sh",
   "crons": [],
+  "functions": {
+    "api/brief/carousel/[userId]/[issueDate]/[page].ts": {
+      "includeFiles": "node_modules/@resvg/resvg-js-linux-x64-gnu/**"
+    }
+  },
   "redirects": [
     { "source": "/docs", "destination": "/docs/documentation", "permanent": false }
   ],