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

tests/server/api/update-contribution.test.js

@@ -83,10 +83,10 @@ describe('update-contribution endpoint — Case 3 (paid→paid)', () => {
     expect(result.message).toBe('Successfully updated contribution level')
   })
 
-  it('annual $5 → $15: calls updateHelcimSubscription with recurringAmount 180', async () => {
+  it('annual $60 → $180: calls updateHelcimSubscription with recurringAmount 180', async () => {
     const mockMember = {
       _id: 'member-2',
-      contributionAmount: 5,
+      contributionAmount: 60,
       helcimSubscriptionId: 'sub-2',
       billingCadence: 'annual',
     }
@@ -97,7 +97,7 @@ describe('update-contribution endpoint — Case 3 (paid→paid)', () => {
     const event = createMockEvent({
       method: 'POST',
       path: '/api/members/update-contribution',
-      body: { contributionAmount: 15, cadence: 'annual' },
+      body: { contributionAmount: 180, cadence: 'annual' },
     })
 
     const result = await handler(event)
@@ -105,16 +105,16 @@ describe('update-contribution endpoint — Case 3 (paid→paid)', () => {
     expect(updateHelcimSubscription).toHaveBeenCalledWith('sub-2', { recurringAmount: 180 })
     expect(Member.findByIdAndUpdate).toHaveBeenCalledWith(
       'member-2',
-      { $set: { contributionAmount: 15 } },
+      { $set: { contributionAmount: 180 } },
       { runValidators: false }
     )
     expect(result.success).toBe(true)
   })
 
-  it('annual $15 → $50: calls updateHelcimSubscription with recurringAmount 600', async () => {
+  it('annual $180 → $600: calls updateHelcimSubscription with recurringAmount 600', async () => {
     const mockMember = {
       _id: 'member-3',
-      contributionAmount: 15,
+      contributionAmount: 180,
       helcimSubscriptionId: 'sub-3',
       billingCadence: 'annual',
     }
@@ -125,7 +125,7 @@ describe('update-contribution endpoint — Case 3 (paid→paid)', () => {
     const event = createMockEvent({
       method: 'POST',
       path: '/api/members/update-contribution',
-      body: { contributionAmount: 50, cadence: 'annual' },
+      body: { contributionAmount: 600, cadence: 'annual' },
     })
 
     await handler(event)
@@ -296,7 +296,7 @@ describe('update-contribution endpoint — Case 1 (free→paid)', () => {
     const event = createMockEvent({
       method: 'POST',
       path: '/api/members/update-contribution',
-      body: { contributionAmount: 15, cadence: 'annual' },
+      body: { contributionAmount: 180, cadence: 'annual' },
     })
 
     const result = await handler(event)
@@ -307,7 +307,7 @@ describe('update-contribution endpoint — Case 1 (free→paid)', () => {
     )
     expect(Member.findByIdAndUpdate).toHaveBeenCalledWith(
       'member-c1',
-      { $set: expect.objectContaining({ billingCadence: 'annual', contributionAmount: 15, helcimSubscriptionId: 'sub-annual' }) },
+      { $set: expect.objectContaining({ billingCadence: 'annual', contributionAmount: 180, helcimSubscriptionId: 'sub-annual' }) },
       { runValidators: false }
     )
     expect(result.success).toBe(true)