Phase 2: share system + analytics feed (backend)

- share_links model/queries; opaque 12-char base62 tokens (PRD §17.1)
- POST /v1/cards/{id}/share — create share link (owner-scoped, modality-validated)
- GET /v1/s/{token} — record modality-attributed event (qr→qr_scan, nfc→nfc_tap)
  and 302-redirect to the profile; unknown/expired → 404
- GET /v1/cards/{id}/share-links — list a card's links
- analytics_events now carry the share_token correlation
- GET /v1/cards/{id}/analytics/feed — chronological event feed (§6.7.2)

6 new integration tests; 73 backend tests total. fmt + clippy clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

cardclaws-backend/crates/cardclaws-api/src/services/mod.rs

@@ -1,5 +1,6 @@
 pub mod analytics_service;
 pub mod auth_service;
 pub mod card_service;
+pub mod share_service;
 pub mod vcard;
 pub mod wallet_service;