docs(adrs): four of the API's crons never run on Vercel

adrs/api-crons-on-vercel.md

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+# Four of the API's crons never run on Vercel
+
+`apps/api/vercel.json` lists five crons. On our Vercel install only one was registered:
+`/internal/sync/google` every five minutes, which is exactly what
+`apps/api/scripts/build-func.mjs` writes into the Build Output `config.json`. Our `crm-api`
+project's Root Directory is the repo root, because `build-func.mjs` writes to
+`<repo>/.vercel/output`, so `apps/api/vercel.json` is never read. Rates, tracking retention
+and archive prune don't run. Telemetry rollup also runs in-process, but on a serverless
+function that's only as often as it boots.
+
+It's worth a look at the Cron Jobs tab on your own `crm-api` project. If it shows one
+entry, it's the same thing.
+
+I'd have `build-func.mjs` copy the `crons` array from `apps/api/vercel.json` into
+`config.json`, so the two can't drift. On a live install that means archive prune starts
+deleting archived records older than the retention setting (180 days by default), and
+tracking retention starts trimming old visits. That's what they're for, but it'll be the
+first time they run, so it's worth a release note.