Report how many documents the reindex actually queues

The index-all trigger returned Boolean(jobId) while the client typed
the field as a number, so the toast read "Indexing true document(s)".
The route now answers with the count of live documents still lacking a
content index, computed from the same live-scoped counts the usage
gauge reads, and an honest 0 when no job was created. OpenAPI schema
updated to the integer shape; the hook's type carries a contract note
so the wire and the toast cannot drift apart again. Route test pins
the number, a component test pins that the toast renders the count and
never the word "true".

Refs #776

docs/api/openapi.yaml

@@ -12792,7 +12792,8 @@ paths:
         immediately; the work runs off-request on the queue.
       responses:
         "200":
-          description: Whether a backfill job was enqueued.
+          description: How many live documents the enqueued backfill still has to index (0 when everything is indexed or no job
+            was created).
           content:
             application/json:
               schema:
@@ -38680,7 +38681,9 @@ components:
       type: object
       properties:
         enqueued:
-          type: boolean
+          type: integer
+          minimum: 0
+          maximum: 9007199254740991
       required:
         - enqueued
       additionalProperties: false