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/lib/api.ts

@@ -160,6 +160,19 @@ export interface PaginatedResponse<T> {
   has_more: boolean;
 }
 
+/**
+ * Server-side aggregates returned alongside `/api/acks` — summed over
+ * the *full* persisted row set (not just the visible page). The
+ * `accepted` / `rejected` / `received` keys match the in-page `totals`
+ * shape so the page can use `data.aggregates` as a drop-in for the
+ * page-local fallback accumulator.
+ */
+export interface AckAggregates {
+  accepted: number;
+  rejected: number;
+  received: number;
+}
+
 /**
  * Lightweight summary used by the GET /api/batches list endpoint. Distinct
  * from the parser's `BatchSummary` (which lives in `@/types` and carries the
@@ -753,21 +766,37 @@ function mapAck(row: RawAckRow): Ack {
   };
 }
 
-async function listAcks(params: { limit?: number } = {}): Promise<PaginatedResponse<Ack>> {
+async function listAcks(
+  params: { limit?: number; offset?: number } = {},
+): Promise<
+  PaginatedResponse<Ack> & {
+    aggregates: AckAggregates;
+  }
+> {
   if (!isConfigured) throw notConfiguredError();
   const query: Record<string, unknown> = {};
   if (params.limit !== undefined) query.limit = params.limit;
+  if (params.offset !== undefined) query.offset = params.offset;
   const body = await authedFetch<{
     items: RawAckRow[];
     total: number;
     returned: number;
     has_more: boolean;
+    aggregates: { accepted_count: number; rejected_count: number; received_count: number };
   }>(`/api/acks${qs(query)}`);
   return {
     items: body.items.map(mapAck),
     total: body.total,
     returned: body.returned,
     has_more: body.has_more,
+    // Adapt the wire-format `*_count` keys to the in-page `totals` shape
+    // so the page can treat server-side and client-side-fallback objects
+    // interchangeably. See useAcks.ts for the matching AckAggregates type.
+    aggregates: {
+      accepted: body.aggregates.accepted_count,
+      rejected: body.aggregates.rejected_count,
+      received: body.aggregates.received_count,
+    },
   };
 }