feat: build core API, fraud engine, notifier, and frontend

Phase 1 — Core API (Go):
- Events, guests, tokens, RSVPs CRUD on PostgreSQL via pgx/v5
- HMAC-signed per-guest tokens with format validation
- Health endpoint with DB ping, slog JSON logging, graceful shutdown

Phase 2 — NATS + Fraud Engine:
- NATS JetStream pub/sub with explicit-ack consumers
- Python/FastAPI fraud engine with heuristic risk scoring
  (fingerprint mismatch, IP change, missing signals, repeated access)
- gRPC sync scoring with 250ms fail-open timeout
- Per-guest baseline tracking; risk bands low/medium/high/block

Phase 3 — Notifications + Frontend:
- Notification worker scaffolding (Twilio/SES stubs, retry/backoff)
- Nuxt 3 frontend with Tailwind dark theme + brand green
- Live monitor via WebSocket with auto-reconnect
- Activity history endpoint backfills monitor with RSVPs +
  scored access checks (including blocked attempts)

UX polish:
- Marketing-friendly landing page (hero mockup, how-it-works,
  features, use cases, testimonials, FAQ, final CTA)
- Animated layered card mockups on landing + new-event page
- Plus-ones stepper, RSVP status badges, filter buttons
- Friendly access-check labels (Verified/Review/Suspicious/Blocked)
- Dashboard hydration fix via ClientOnly wrapper

Infrastructure:
- docker-compose for full local dev (postgres, nats, api,
  fraud-engine, notifier, frontend)
- Multi-stage Dockerfiles, non-root UID 1000
- Integration tests with testcontainers-go

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/api/users.go

@@ -0,0 +1,55 @@
+package api
+
+import (
+	"encoding/json"
+	"errors"
+	"net/http"
+	"net/mail"
+
+	"github.com/alchemistkay/guestguard/internal/domain"
+	"github.com/alchemistkay/guestguard/internal/storage"
+)
+
+type userHandler struct {
+	repo *storage.UserRepo
+}
+
+type upsertUserRequest struct {
+	Email string `json:"email"`
+	Name  string `json:"name"`
+}
+
+// POST /users — idempotent: returns the existing user if the email already
+// exists, creates one otherwise. This keeps the demo flow simple without
+// requiring real auth.
+func (h *userHandler) upsert(w http.ResponseWriter, r *http.Request) {
+	var req upsertUserRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid json")
+		return
+	}
+	if _, err := mail.ParseAddress(req.Email); err != nil {
+		writeError(w, http.StatusBadRequest, "email is invalid")
+		return
+	}
+	if req.Name == "" {
+		writeError(w, http.StatusBadRequest, "name is required")
+		return
+	}
+
+	u, err := h.repo.Create(r.Context(), req.Email, req.Name)
+	if err == nil {
+		writeJSON(w, http.StatusCreated, u)
+		return
+	}
+	if errors.Is(err, domain.ErrEmailTaken) {
+		existing, getErr := h.repo.GetByEmail(r.Context(), req.Email)
+		if getErr != nil {
+			writeError(w, http.StatusInternalServerError, "failed to load user")
+			return
+		}
+		writeJSON(w, http.StatusOK, existing)
+		return
+	}
+	writeError(w, http.StatusInternalServerError, "failed to create user")
+}