fix(brief): use wildcard glob in vercel.json functions key (PR #3204 follow-up) (#3206)

* fix(brief): use wildcard glob in vercel.json functions key

PR #3204 shipped the right `includeFiles` value but the WRONG key:

  "api/brief/carousel/[userId]/[issueDate]/[page].ts"

Vercel's `functions` config keys are micromatch globs, not literal
paths. Bracketed segments like `[userId]` are parsed as character
classes (match any ONE character from {u,s,e,r,I,d}), so my rule
matched zero files and `includeFiles` was silently ignored. Post-
merge probe still returned HTTP 500 FUNCTION_INVOCATION_FAILED on
every request. Build log shows zero mentions of `carousel` or
`resvg` — corroborates the key never applied.

Fix: wildcard path segments.

  "api/brief/carousel/**"

Matches any file under the carousel route dir. Since the only
deployed file there is the dynamic-segment handler, the effective
scope is identical to what I originally intended.

Added a second regression test that sweeps every functions key and
fails loudly if any bracketed segment slips back in. Guards against
future reverts AND against anyone copy-pasting the literal route
path without realising Vercel reads it as a glob.

23/23 deploy-config tests pass (was 22, +1 new guard).

* Address Greptile P2: widen bracket-literal guard regex

Greptile spotted that `/\[[A-Za-z]+\]/` only matches purely-alphabetic
segment names. Real-world Next.js routes often use `[user_id]`,
`[issue_date]`, `[page1]`, `[slug2024]` — none flagged by the old
regex, so the guard would silently pass on the exact kind of
regression it was written to catch.

Widened to `/\[[A-Za-z][A-Za-z0-9_]*\]/`:
  - requires a leading letter (so legit char classes like `[0-9]`
    and `[!abc]` don't false-positive)
  - allows letters, digits, underscores after the first char
  - covers every Next.js-style dynamic-segment name convention

Also added a self-test that pins positive cases (userId, user_id,
issue_date, page1, slug2024) and negative cases (the actual `**`
glob, `[0-9]`, `[!abc]`) so any future narrowing of the regex
breaks CI immediately instead of silently re-opening PR #3206.

24/24 deploy-config tests pass (was 23, +1 new self-test).

vercel.json

@@ -2,7 +2,7 @@
   "ignoreCommand": "bash scripts/vercel-ignore.sh",
   "crons": [],
   "functions": {
-    "api/brief/carousel/[userId]/[issueDate]/[page].ts": {
+    "api/brief/carousel/**": {
       "includeFiles": "node_modules/@resvg/resvg-js-linux-x64-gnu/**"
     }
   },