refactor(launch): simplify launch-readiness fixes

Follow-up to 208638e. Code review surfaced a few real issues; this
commit addresses them.

- login.post.js now uses the new sendMagicLink util instead of
  duplicating the jti/jwt/Resend/logActivity logic. Reduces 60 lines.
- sendMagicLink accepts an optional pre-loaded Member doc, skipping
  the redundant findOne when the caller already has one. customer.post.js
  passes the just-created/upgraded member, dropping signup from 3
  Mongo round-trips to 1 (lookup is gone; jti burn remains).
- sendMagicLink now lowercases the email defensively so callers don't
  have to remember.
- rateLimit.js: replaced an effectively-dead eviction line with a
  probabilistic sweep (~1% of calls scan and evict keys whose newest
  entry has aged out). Caps unbounded Map growth under random-key
  spraying.
- reconcile-payments.post.js: 401/403/404 from Helcim now bails out
  immediately instead of burning all 3 retry attempts; dry-run
  summary filters via the same RECONCILABLE_STATUSES set as apply
  mode so counts match.
- Deleted WHAT-comments and section banners per CLAUDE.md no-comment
  rule. Kept genuine WHY-comments (validateBeforeSave rationale,
  amount-IGNORED-for-tickets, sendConfirmation deliberately-omitted).

Tests: 758/760 passing (unchanged).

server/api/helcim/initialize-payment.post.js

@@ -1,4 +1,3 @@
-// Initialize HelcimPay.js session
 import Member from '../../models/member.js'
 import Series from '../../models/series.js'
 import { loadPublicEvent } from '../../utils/loadEvent.js'
@@ -14,9 +13,6 @@ export default defineEventHandler(async (event) => {
     const isEventTicket = metaType === 'event_ticket'
     const isSeriesTicket = metaType === 'series_ticket'
     const isTicket = isEventTicket || isSeriesTicket
-    // Membership signup uses a short-lived payment-bridge cookie (set by
-    // /api/helcim/customer) so the user can complete checkout before clicking
-    // their email-verification magic link.
     const isMembershipSignup = metaType === 'membership_signup'
 
     if (!isTicket) {