feat(account): stamp last-login time + IP on cold app-load

The profile GET (fetched once per cold app-load by the SPA) stamps accounts
.last_login_at/.last_login_ip, throttled to at most once per hour per account
(best-effort, never blocks the read). IP from the gateway-forwarded X-Forwarded-For.
Feeds the account-deletion dossier. Integration test covers the throttle.

backend/internal/server/handlers_user.go

@@ -25,6 +25,10 @@ func (s *Server) handleProfile(c *gin.Context) {
 		s.abortErr(c, err)
 		return
 	}
+	// The SPA fetches the profile once per cold app-load, so stamp the account's last
+	// login time and client IP here (throttled to at most once an hour). Best-effort: it
+	// feeds the deletion dossier, never blocks the profile read.
+	_ = s.accounts.StampLastLogin(c.Request.Context(), uid, clientIP(c))
 	c.JSON(http.StatusOK, s.profileResponse(c.Request.Context(), acc))
 }