feat(labs): make the lab-scan limit operator-tunable

A self-hoster scanning a stack of lab reports ran into a fixed six scans
an hour with no way to raise it. LABS_OCR_LIMIT_PER_HOUR now sets the
per-user ceiling (default 6, clamped to 1-1000, falling back to the
default when unset or not a number), the same shape as
DOCUMENT_AI_LIMIT_PER_HOUR. It is on the compose whitelist, in the
production example, the env manifest and the AI provider guide.

The route also charged the slot before it had looked at the request, so
a malformed body, an unsupported or oversized file, a PDF that could not
be rendered or a scan the daily budget refused still used up the hour.
Those failures hand the slot back now; a scan that reached the provider
keeps it. The 429 carries meta.retryAt. Refs #985

docs/api/openapi.yaml

@@ -13489,8 +13489,10 @@ paths:
         provider; ≤ 12 MiB) is run through the user's vision-capable provider; the upload lives in memory only and is
         never persisted or logged. TEXT (`application/json`, opt-in local OCR): the browser OCR's the image
         (tesseract.js) and POSTs `{ mode: \"text\", text }` — only the extracted text reaches the server, so a text-only
-        provider (e.g. ChatGPT-OAuth) reaches the same review/commit flow. Both modes are gated by AI consent, a 6/hour
-        rate bucket, and the per-day token budget, and return proposed rows for the mandatory human review screen —
+        provider (e.g. ChatGPT-OAuth) reaches the same review/commit flow. Both modes are gated by AI consent, a
+        per-user hourly rate bucket (default 6, operator-tunable via `LABS_OCR_LIMIT_PER_HOUR`; a slot is only consumed
+        when the scan reaches the provider, and the 429 carries `Retry-After`, the `X-RateLimit-*` triple and
+        `meta.retryAt`), and the per-day token budget, and return proposed rows for the mandatory human review screen —
         nothing is written. Extracted content is treated as untrusted (prompt-injection); the review step is the safety
         boundary."
       requestBody: