fix(labs): say when a scanned PDF was read from its first pages only

A PDF longer than the pages a scan renders was read from its first ten
pages, and the cut was recorded only in the wide event. The person checking
the extracted rows had no way to know that values on later pages were never
read.

The rasterizer now returns the PDF's page count, the extract response
carries pageCoverage { read, total } when the cap applied, and the review
screen says how many of the pages were read.

docs/api/openapi.yaml

@@ -63012,6 +63012,23 @@ components:
           type: array
           items:
             $ref: "#/components/schemas/OcrExtractedRow"
+        pageCoverage:
+          description: "Present only when a PDF ran past the pages a scan reads: the rows come from the first `read` of `total`
+            pages. Absent when the whole document was read."
+          type: object
+          properties:
+            read:
+              type: integer
+              exclusiveMinimum: 0
+              maximum: 9007199254740991
+            total:
+              type: integer
+              exclusiveMinimum: 0
+              maximum: 9007199254740991
+          required:
+            - read
+            - total
+          additionalProperties: false
       required:
         - reportDate
         - providerType