feat(slack): answer the order nudge in Slack, without opening the app

Being sent to a web app to answer "was this shared?" is enough friction
that the question stops getting answered — which is the exact failure the
nudge exists to prevent. So the buttons now act in place: pressing
"Shared 50/50" writes the transaction_splits row and edits the message,
and the app is never opened.

Slack does NOT reach this route directly. It posts to an n8n webhook that
forwards the raw body and signature headers here (user's suggestion).
That is the better shape: n8n already terminates public webhooks, so the
app keeps its blanket OAuth chain and gains no internet-facing
unauthenticated route, and no Traefik change is needed. n8n cannot verify
the signature itself — its Code sandbox has no `require`, so no `crypto`.

Two independent gates, both failing closed: the shared x-ingest-token
(came from n8n) and Slack's v0 signature over the raw body (came from
Slack, not replayed within 5 minutes). An unset signing secret rejects
everything rather than waving it through, because the alternative turns a
misconfigured deploy into an open write endpoint.

An unmapped Slack user is refused rather than defaulted to the owner. In
a two-person household a wrong attribution is not a rounding error, it is
the other person's opinion recorded under your name.

Block Kit is built in the app (ingest returns slack_blocks) rather than
in n8n expressions: a template string is untestable, and this shape has
to stay in step with what the interactive endpoint re-renders after each
press. Null when there is no transaction yet — a card-settled order is
parked until its statement arrives, so there is nothing to split or rate.

Also: /transactions now honours ?q=, so the link lands on the row instead
of the top of an unfiltered ledger.

src/app/api/orders/ingest/route.ts

@@ -12,6 +12,7 @@ import {
   type MessageMeta,
 } from "@/lib/order-ingestion";
 import { merchantVerdict } from "@/lib/order-reviews";
+import { nudgeBlocks } from "@/lib/slack-blocks";
 
 /**
  * Machine ingest endpoint for order receipts.
@@ -99,6 +100,24 @@ export async function POST(req: NextRequest) {
         counts: verdict.counts,
         last_note: verdict.history.find((h) => h.note)?.note ?? null,
       },
+      // The nudge message is built here, not in n8n expressions: Block Kit in a
+      // template string is untestable, and this shape has to stay in step with
+      // what /api/slack/interactive renders after a button press. Null when
+      // there is no transaction to act on — a card-settled order is parked
+      // until its statement arrives, so there is nothing yet to split or rate.
+      slack_blocks:
+        result.transactionId && !result.skipped
+          ? nudgeBlocks({
+              transactionId: result.transactionId,
+              merchant: order.merchant_name,
+              currency: order.currency,
+              total: Number(order.totals.total_charged),
+              isFamily: order.is_family,
+              shared: false,
+              warn: verdict?.warn ?? false,
+              warnNote: verdict?.history.find((h) => h.note)?.note ?? null,
+            })
+          : null,
     });
   } catch (e) {
     // Not a receipt: promotions, delivery updates, adjustment and refund