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

docker-compose.yml

@@ -290,6 +290,12 @@ x-healthlog-env: &healthlog_env
   # (the `environment:` block is a whitelist). See
   # docs/integrations/ai-providers.md.
   DOCUMENT_AI_LIMIT_PER_HOUR: "${DOCUMENT_AI_LIMIT_PER_HOUR:-}"
+  # Per-user hourly ceiling for lab-report scans (photo/PDF and browser-OCR
+  # text share one bucket; optional; default 6, clamped 1-1000). A scan that
+  # fails before the provider is called hands its slot back. Listed here so an
+  # operator value in .env reaches the container. See
+  # docs/integrations/ai-providers.md.
+  LABS_OCR_LIMIT_PER_HOUR: "${LABS_OCR_LIMIT_PER_HOUR:-}"
   # v1.17.1 — SMTP transport for the email notification channel (optional,
   # all-or-none: HOST + PORT + FROM enable it; USER/PASS optional; SECURE
   # toggles implicit TLS). Listed here so operator values in .env reach the