Stage 16: deploy infra & test contour

- backend + gateway multi-stage distroless Dockerfiles; the gateway embeds and
  serves the SPA at / and /telegram/ via go:embed (committed dist placeholder,
  real build baked in by the image's node stage)
- deploy/docker-compose.yml: backend + gateway + Postgres + Telegram connector
  (VPN sidecar) + OTel Collector + Prometheus (15d) + Tempo (72h) + Grafana,
  fronted by a caddy owning a single /_gm Basic-Auth (admin console + Grafana
  subpath); inter-service on a private network, only caddy on the edge network
- new metrics: backend accounts_created_total{kind} (robots excluded) and an
  in-memory gateway active_users{window=24h,7d} gauge
- CI: single .gitea/workflows/ci.yaml (unit/integration/ui + a gated test-contour
  deploy) on the new feature/* -> development -> master branch model; the old
  go-unit/integration/ui-test workflows are folded in; the connector-scoped
  compose is retired (superseded by deploy/)
- docs: ARCHITECTURE §11/§12/§13, root + gateway READMEs, CLAUDE.md branching,
  PLAN.md (stage 16 done + refinements + Stage 17 forward-notes)

deploy/grafana/dashboards/edge-ux.json

@@ -0,0 +1,39 @@
+{
+  "uid": "scrabble-edge",
+  "title": "Scrabble — Edge / UX",
+  "tags": ["scrabble"],
+  "timezone": "",
+  "schemaVersion": 39,
+  "version": 1,
+  "refresh": "30s",
+  "time": { "from": "now-6h", "to": "now" },
+  "panels": [
+    {
+      "type": "timeseries",
+      "title": "Edge request rate by message type",
+      "gridPos": { "h": 8, "w": 12, "x": 0, "y": 0 },
+      "fieldConfig": { "defaults": { "unit": "reqps" }, "overrides": [] },
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "targets": [{ "refId": "A", "expr": "sum(rate(edge_request_duration_count[5m])) by (message_type)", "legendFormat": "{{message_type}}" }]
+    },
+    {
+      "type": "timeseries",
+      "title": "Edge p95 latency",
+      "gridPos": { "h": 8, "w": 12, "x": 12, "y": 0 },
+      "fieldConfig": { "defaults": { "unit": "s" }, "overrides": [] },
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "targets": [
+        { "refId": "A", "expr": "histogram_quantile(0.95, sum(rate(edge_request_duration_bucket[5m])) by (le))", "legendFormat": "p95" },
+        { "refId": "B", "expr": "histogram_quantile(0.50, sum(rate(edge_request_duration_bucket[5m])) by (le))", "legendFormat": "p50" }
+      ]
+    },
+    {
+      "type": "timeseries",
+      "title": "Edge requests by result",
+      "gridPos": { "h": 8, "w": 24, "x": 0, "y": 8 },
+      "fieldConfig": { "defaults": { "unit": "reqps" }, "overrides": [] },
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "targets": [{ "refId": "A", "expr": "sum(rate(edge_request_duration_count[5m])) by (result)", "legendFormat": "{{result}}" }]
+    }
+  ]
+}