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/utils/slack.ts

@@ -90,6 +90,7 @@ export class SlackService {
     memberEmail: string,
     circle: string,
     contributionAmount: number,
+    cadence: string = 'monthly',
     invitationStatus: string = "manual_invitation_required",
   ): Promise<void> {
     try {
@@ -148,7 +149,7 @@ export class SlackService {
               },
               {
                 type: "mrkdwn",
-                text: `*Contribution:*\n$${contributionAmount}/month`,
+                text: `*Contribution:*\n$${contributionAmount}/${cadence === 'annual' ? 'yr' : 'mo'}`,
               },
             ],
           },