fix(server): treat contributionAmount as cadence-unit (drop ×12)

ContributionAmountField now emits cadence-unit values (180 for $180/yr,
15 for $15/mo). Server endpoints were still multiplying annual by 12,
which would have charged $2160/yr instead of $180/yr after the form
ports in Tasks 2–3.

- helcim/subscription.post.js: recurringAmount = contributionAmount
  (no more × 12 for annual)
- members/update-contribution.post.js: same drop in both Case 1
  (free→paid) and Case 3 (paid→paid)
- slack.ts notifyNewMember: new positional `cadence` param so the
  Slack notification suffix renders /yr or /mo instead of hardcoded
  /month; all three call sites updated to pass member.billingCadence
- tests updated to match the new contract:
  - helcim-subscription.test.js: annual tests now send the cadence-
    unit amount (180, 600) and expect the same recurringAmount
  - update-contribution.test.js: annual Case 1 and Case 3 tests
    updated likewise

server/api/members/update-contribution.post.js

@@ -88,7 +88,7 @@ export default defineEventHandler(async (event) => {
             dateActivated: new Date().toISOString().split("T")[0],
             paymentPlanId: parseInt(paymentPlanId),
             customerCode,
-            recurringAmount: cadence === 'annual' ? newAmount * 12 : newAmount,
+            recurringAmount: newAmount,
             paymentMethod: "card",
           },
           idempotencyKey,
@@ -217,7 +217,7 @@ export default defineEventHandler(async (event) => {
       try {
         const subscriptionData = await updateHelcimSubscription(
           member.helcimSubscriptionId,
-          { recurringAmount: memberCadence === 'annual' ? newAmount * 12 : newAmount }
+          { recurringAmount: newAmount }
         );
 
         await Member.findByIdAndUpdate(