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  compose is retired (superseded by deploy/)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)

backend/internal/account/account.go

@@ -93,12 +93,14 @@ type Identity struct {
 
 // Store is the Postgres-backed query surface for accounts and identities.
 type Store struct {
-	db *sql.DB
+	db      *sql.DB
+	metrics *accountMetrics
 }
 
-// NewStore constructs a Store wrapping db.
+// NewStore constructs a Store wrapping db. Metrics default to a no-op meter until
+// SetMetrics installs the real one during startup wiring.
 func NewStore(db *sql.DB) *Store {
-	return &Store{db: db}
+	return &Store{db: db, metrics: defaultAccountMetrics()}
 }
 
 // ProvisionByIdentity returns the account bound to (kind, externalID), creating
@@ -331,6 +333,11 @@ func (s *Store) create(ctx context.Context, kind, externalID string, seed provis
 	if err != nil {
 		return Account{}, fmt.Errorf("account: create for identity (%s, %s): %w", kind, externalID, err)
 	}
+	// Count genuinely new durable accounts; robots are a fixed provisioned pool,
+	// not users, so they are excluded.
+	if kind != KindRobot {
+		s.metrics.recordCreated(ctx, kind)
+	}
 	return created, nil
 }
 
@@ -355,6 +362,7 @@ func (s *Store) ProvisionGuest(ctx context.Context) (Account, error) {
 	if err := stmt.QueryContext(ctx, s.db, &row); err != nil {
 		return Account{}, fmt.Errorf("account: provision guest: %w", err)
 	}
+	s.metrics.recordCreated(ctx, kindGuest)
 	return modelToAccount(row), nil
 }