sub-issue: Create export-contacts edge function

[malek10xdev]

Relates to #2750 — Phase 2 migration.

Goal

Move contact export (CSV, VCard, Google Contacts) from Express backend to a new export-contacts edge function.

Endpoint

Method	Path	Purpose
POST	/:type	Export contacts (csv/vcard/google_contacts) with credit verification

Key challenges

• Billing flow: Port credit verification including 402 (no credits), 266 (confirm partial), 206 (partial content) responses
• File generation: CSV and VCard generators are pure functions, portable
• Google Contacts API: Uses OAuth access token from mining sources
• Rate limiter: Replace express-rate-limit (100 req/15min)

Files to create

• supabase/functions/export-contacts/index.ts — Hono app
• supabase/functions/export-contacts/formats/ — CSV, VCard, Google Contacts generators

Files to modify

• frontend/src/components/mining/buttons/ExportContacts.vue

Files to remove

• backend/src/routes/contacts.routes.ts:29-34
• backend/src/controllers/contacts.controller.ts:207-273
• backend/src/services/export/ (entire directory)

Acceptance criteria

• CSV export works from edge function with billing
• VCard export works from edge function with billing
• Google Contacts export works with OAuth access
• Credit verification (402/266/206) responses work correctly
• Frontend calls edge function directly
• Existing tests pass