feat(slack): ask the other person too, and put rating before sharing

Two halves of the same requirement, one of which was quietly missing.

Sharing split the money but never reached her: she is not in #smarthome,
so the card whose caption said "both verdicts welcome" was one she could
not see. Now a share DMs her a card of her own. A DM rather than adding
her to the channel, so her surface stays "orders that concern me"
instead of the whole house's ops feed. She was already in SLACK_USER_MAP,
so her press files under participant 4.

Only on the press that turns sharing ON, and only when someone else did
the sharing. Re-notifying on every later rating press would turn one
shared meal into a stream of DMs, which is how a nudge gets muted.

Her card carries no share button: she is being told it was shared, not
asked to decide, and two people toggling one split from separate copies
of a card is a race with no upside.

The app still holds no Slack bot token — it returns a notify instruction
and n8n sends it, the same shape as the modal open. If SLACK_USER_MAP has
no id for her the DM is skipped silently: the split is correct and
complete either way, and failing the press over an unaddressable nudge
would be the worse trade.

Card reordered to rate -> details -> share. You judge the food, then
decide who pays for it; asking "was this shared?" first inverts the
order a person thinks in. The status caption moved under the share button
it describes rather than sitting orphaned mid-card.

src/app/api/slack/interactive/route.ts

@@ -1,7 +1,11 @@
 import { NextRequest, NextResponse } from "next/server";
 import { queryRaw, queryRow } from "@/lib/db";
-import { verifySlackSignature, participantForSlackUser } from "@/lib/slack-verify";
-import { nudgeBlocks, detailsModal } from "@/lib/slack-blocks";
+import {
+  verifySlackSignature,
+  participantForSlackUser,
+  slackUserForParticipant,
+} from "@/lib/slack-verify";
+import { nudgeBlocks, detailsModal, partnerNudgeBlocks } from "@/lib/slack-blocks";
 import {
   RATINGS,
   OWNER_PARTICIPANT_ID,
@@ -105,14 +109,50 @@ export async function POST(req: NextRequest) {
   const state = await nudgeState(transactionId);
   if (!state) return NextResponse.json({ text: "That order is no longer in the ledger." });
 
+  // Ask the other person for their verdict, but only on the press that turned
+  // sharing ON — and only when it was someone else who shared it with them.
+  // Re-notifying on every subsequent rating press would make one shared meal
+  // a stream of DMs, which is how a useful nudge becomes muted.
+  const notify =
+    verb === "share" && state.shared && participantId !== SECOND_CONSUMER_ID
+      ? buildPartnerNotify(state, payload.user?.name)
+      : null;
+
   return NextResponse.json({
     replace_original: true,
     blocks: nudgeBlocks(state),
     // Notification text for clients that cannot render blocks.
     text: `${state.merchant} — ${state.currency} ${state.total.toFixed(2)}`,
+    ...(notify ? { notify } : {}),
   });
 }
 
+/**
+ * The DM payload for the other person, or null if we cannot address them.
+ *
+ * Returns an instruction rather than sending: the app holds no Slack bot token,
+ * so n8n — which already has the credential — makes the call. Same shape as the
+ * modal open.
+ *
+ * Null when SLACK_USER_MAP has no Slack id for the second consumer. Silent
+ * rather than an error: the split is still correct and complete, and failing
+ * the whole press because a DM could not be addressed would be worse than the
+ * missing nudge.
+ */
+function buildPartnerNotify(
+  state: Awaited<ReturnType<typeof nudgeState>>,
+  sharerName?: string
+) {
+  if (!state) return null;
+  const slackUser = slackUserForParticipant(SECOND_CONSUMER_ID);
+  if (!slackUser) return null;
+  return {
+    user: slackUser,
+    text: `${state.merchant} — shared with you 50/50`,
+    blocks: partnerNudgeBlocks(state, sharerName || "It was"),
+  };
+}
+
 /** The modal view, pre-filled with whatever this person already said. */
 async function buildDetailsModal(transactionId: number, participantId: number) {
   const row = await queryRow<{