R2: load-test harness + contour resource observability

New scrabble/loadtest module (the pre-release stress harness): seeds 1000 guest +
10000 durable accounts with pre-created sessions directly in Postgres (token hash
matches backend/internal/session), drives virtual players through the edge protocol
(real 2-4p games assembled via invitations, mid-ranked legal moves generated locally
by the embedded scrabble-solver — the edge carries no board, so the client replays
history), plus nudge/chat/check-word/draft/profile/stats and a gateway-hammer that
verifies the rate limiter. Prints a trip-report summary (per-op latency percentiles,
result codes, live-event tally). Go unit tests cover the pure pieces; the DAWG-backed
move test runs under BACKEND_DICT_DIR.

Contour: add cAdvisor + postgres_exporter + a 'Scrabble - Resources' Grafana
dashboard and the two Prometheus scrape jobs, for the R2/R7 stress-run resource
baseline.

CI: gate ./loadtest/... (path filter + vet/build/test). Docs: TESTING, ARCHITECTURE,
project CLAUDE repo layout.

loadtest/internal/scenario/hammer.go

@@ -0,0 +1,45 @@
+package scenario
+
+import (
+	"context"
+	"sync"
+	"time"
+
+	"scrabble/loadtest/internal/seed"
+)
+
+// HammerConfig parameterises the gateway-hammer: how many concurrent callers and for
+// how long to deliberately exceed the per-user rate limit from a single account.
+type HammerConfig struct {
+	Workers  int
+	Duration time.Duration
+}
+
+// DefaultHammer returns a hammer that comfortably exceeds the 300/min per-user limit.
+func DefaultHammer() HammerConfig {
+	return HammerConfig{Workers: 20, Duration: 15 * time.Second}
+}
+
+// Hammer drives games.list from a single account far above the per-user rate limit to
+// verify the limiter holds — rejections surface as the "rate_limited" code — and to
+// measure its cost. Every call is recorded under "hammer:games.list" so the report
+// shows the ok/rate_limited split and the rejection latency separately from the
+// realistic traffic.
+func (d *Driver) Hammer(ctx context.Context, acc seed.Account, cfg HammerConfig) {
+	runCtx, cancel := context.WithTimeout(ctx, cfg.Duration)
+	defer cancel()
+	d.log.Info("gateway-hammer", "workers", cfg.Workers, "duration", cfg.Duration)
+	var wg sync.WaitGroup
+	for w := 0; w < cfg.Workers; w++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for runCtx.Err() == nil {
+				t0 := time.Now()
+				_, code, _ := d.edge.GamesList(runCtx, acc.Token)
+				d.rec.Record("hammer:games.list", code, time.Since(t0))
+			}
+		}()
+	}
+	wg.Wait()
+}