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

scripts/env-manifest.json

@@ -137,6 +137,10 @@
         {
           "name": "DOCUMENT_AI_LIMIT_PER_HOUR",
           "purpose": "Per-user hourly ceiling for the document-AI actions (read/index, extract, suggest, summarise share one bucket). Default 6, clamped to 1-1000."
+        },
+        {
+          "name": "LABS_OCR_LIMIT_PER_HOUR",
+          "purpose": "Per-user hourly ceiling for lab-report scans (photo/PDF and browser-OCR text share one bucket). Default 6, clamped to 1-1000."
         }
       ]
     },