feat(server): confirm-link endpoint for the one-tap deeplink

Add POST /internal/sessions/email/confirm-link: it verifies a deeplink token via
ConfirmByToken and, for a login, mints a session (the deeplink page signs in with
it); for a link, attaches the confirmed email and reports "confirmed" or
"merge_required" (the app drives the interactive merge). The token, not a request
session, is the authorization. Add LinkConfirmation.IsLogin() and integration tests
for the login, link and merge branches (the token is read from the mailed link).
The gateway RPC, live event and SPA route follow.

backend/internal/server/handlers.go

@@ -31,6 +31,7 @@ func (s *Server) registerRoutes() {
 		in.POST("/sessions/guest", s.handleGuestAuth)
 		in.POST("/sessions/email/request", s.handleEmailRequest)
 		in.POST("/sessions/email/login", s.handleEmailLogin)
+		in.POST("/sessions/email/confirm-link", s.handleEmailConfirmLink)
 		in.POST("/sessions/resolve", s.handleResolveSession)
 		in.POST("/sessions/revoke", s.handleRevokeSession)
 		// Out-of-app push routing for the platform side-service: the