hotfix(acks): paginate /api/acks and surface server-side aggregates

The Acks page silently capped at 100 of 1056 rows in the operator's
DB: the eyebrow read `${items.length} on file` (not the server's
`total`), the KPI strip summed from the 100 visible items, and the
endpoint accepted no `offset` — so the user had no signal that 956
more acks existed. Same class of bug on the Activity page (cap=200,
no 'X of Y' hint).

Backend
- /api/acks (acks.py): add `offset`, bump `le=1000`→`le=5000`,
  slice `rows[offset:offset+limit]`, return server-side
  `aggregates` (accepted/rejected/received summed over the full
  row set, not the page) so the KPI strip reflects every persisted
  999 instead of just the visible 50.
- /api/activity list + stream (api.py): bump `le=500`→`le=5000` so
  the page can ask for a denser snapshot.
- /api/277ca-acks (api.py): bump `le=1000`→`le=5000` for
  consistency.
- /api/ta1-acks: left at `le=1000` — TA1s aren't shipped today and
  the structural fix (offset + aggregates) wasn't applied, so a
  larger cap would just make the same latent silent-failure easier
  to hit. (TODO: fold in the same shape when Gainwell starts
  shipping TA1s.)

Frontend
- listAcks (api.ts): accept `offset`, surface `aggregates`,
  adapt wire `*_count` keys to the in-page
  `accepted`/`rejected`/`received` shape so the page can use
  `data.aggregates` as a drop-in for the page-local fallback
  accumulator.
- useAcks (hooks): pass `offset` through; return type carries
  `aggregates`.
- Acks.tsx: add `page` state (PAGE_SIZE=50), use `data.total`
  for eyebrow + watermark (not `items.length`), use
  `data.aggregates` for the KPI strip (with in-page fallback
  accumulator on first paint), render `<Pagination>` when
  `totalCount > PAGE_SIZE`. Footer row reads "N rows on file"
  instead of "N rows".
- ActivityLog.tsx: bump `limit: 200`→`limit: 500`, eyebrow reads
  "Activity · showing N most recent" to make the bounded-window
  semantics honest (the endpoint doesn't expose a true total — it
  reports events matching the current kind/since filter, capped at
  the request limit).

Tests
- test_acks.py: 4 new tests pin the fix:
  1. `offset` walks the full set; `has_more` flips at the
     boundary.
  2. `aggregates` reflects the full row set, not the page (the
     silent-failure pin) — and stays stable across page slices.
  3. `limit` cap of 5000 is enforced (422 above it).
  4. `offset` past the end returns an empty page with stable
     aggregates (a stale UI page state across a row count change
     must not 500 or zero the KPIs).

Live smoke-verified: /api/acks?limit=2&offset=0 vs ?offset=2 return
the expected row slices, aggregates stable at 15/10/15 for 5 seeded
rows, /api/acks?limit=10000 rejected with 422.

Triage note: the TA1 section (`Ta1AcksSection`, lines 609-616 of
Acks.tsx) has the same latent silent-failure pattern (page-sums
KPIs, no offset on /api/ta1-acks). Left untouched because the
empty-state copy says Gainwell doesn't ship TA1s today and the
larger structural fix belongs in a follow-up.

src/pages/ActivityLog.tsx

@@ -68,7 +68,13 @@ export function ActivityLog() {
   const { data, isLoading, isError, error, refetch } = useActivity({
     kind: apiKind,
     since: sinceIso,
-    limit: 200,
+    // 500 keeps the wire size manageable (and the operator can rely
+    // on the live tail to surface new events as they land). The
+    // activity list endpoint doesn't expose a true total — `data.total`
+    // reflects events matching the current kind/since filter, capped
+    // at the request limit — so the eyebrow below uses "Showing N
+    // most recent" instead of "X of Y" to avoid a misleading ratio.
+    limit: 500,
   });
 
   const allItems = data?.items ?? [];
@@ -127,7 +133,11 @@ export function ActivityLog() {
   return (
     <div className="space-y-6 lg:space-y-8 animate-fade-in">
       <PageHeader
-        eyebrow="Activity"
+        eyebrow={
+          allItems.length > 0
+            ? `Activity · showing ${allItems.length.toLocaleString()} most recent`
+            : "Activity"
+        }
         title={<>Activity <span className="display italic text-muted-foreground">log</span></>}
         subtitle="Every claim submission, denial, payment, and provider event, in reverse chronological order. Auto-refreshes every 30s."
         actions={