refactor(affiliate): tighten DI and harden inviter code validation

- Drop SetAffiliateService setters and ProvideAuthService /
  ProvidePaymentService / ProvideUserHandler wrappers in favor of direct
  Wire constructor injection. AffiliateService has no back-edge to
  Auth/Payment/User, so the indirection was never required.
- Change RegisterWithVerification's variadic affiliateCode to a fixed
  parameter; adjust all call sites.
- Validate aff_code length and charset in BindInviterByCode before any
  DB lookup, eliminating timing-side-channel and useless DB roundtrips
  on malformed input.
- Make affiliate cache invalidation synchronous; surface Redis errors
  via the project logger instead of swallowing them in a detached
  goroutine.
- Add an integration test guarding cross-layer tx propagation in
  AccrueQuota and a unit test pinning the aff_code format rules.

backend/internal/service/auth_oauth_email_flow_test.go

@@ -137,6 +137,7 @@ func newOAuthEmailFlowAuthService(
 		nil,
 		nil,
 		nil,
+		nil,
 	)
 }