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/hooks/useAcks.ts

@@ -1,5 +1,5 @@
 import { useQuery } from "@tanstack/react-query";
-import { api, type PaginatedResponse } from "@/lib/api";
+import { api, type AckAggregates, type PaginatedResponse } from "@/lib/api";
 import type { Ack } from "@/types";
 
 /**
@@ -7,9 +7,14 @@ import type { Ack } from "@/types";
  * intentionally has no in-memory fallback (there is no zustand
  * sample-data path for ACKs in v1 — the UI is empty until the
  * backend serves real rows).
+ *
+ * The response carries `aggregates` summed over the *full* row set
+ * (not just the visible page) so the KPI strip on the Acks page
+ * reflects every persisted 999 — without this, totals silently
+ * under-report once the row count exceeds the page size.
  */
-export function useAcks(params: { limit?: number } = {}) {
-  return useQuery<PaginatedResponse<Ack>>({
+export function useAcks(params: { limit?: number; offset?: number } = {}) {
+  return useQuery<PaginatedResponse<Ack> & { aggregates: AckAggregates }>({
     queryKey: ["acks", params],
     queryFn: () => api.listAcks(params),
     enabled: api.isConfigured,