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/respond.go

@@ -0,0 +1,26 @@
+package api
+
+import (
+	"encoding/json"
+	"log/slog"
+	"net/http"
+)
+
+type errorBody struct {
+	Error string `json:"error"`
+}
+
+func writeJSON(w http.ResponseWriter, status int, body any) {
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	w.WriteHeader(status)
+	if body == nil {
+		return
+	}
+	if err := json.NewEncoder(w).Encode(body); err != nil {
+		slog.Error("write json", "err", err)
+	}
+}
+
+func writeError(w http.ResponseWriter, status int, msg string) {
+	writeJSON(w, status, errorBody{Error: msg})
+}