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.

CLAUDE.md

@@ -126,8 +126,9 @@ backend/                     # module scrabble/backend
 docs/  .gitea/workflows/  PLAN.md  CLAUDE.md  README.md
 gateway/  ui/  pkg/              # added by their stages
 platform/telegram/               # Telegram connector side-service (Stage 9): bot + gRPC API
-backend/Dockerfile gateway/Dockerfile platform/telegram/Dockerfile  # multi-stage distroless (Stage 16)
-deploy/                          # docker-compose + caddy + otelcol/prometheus/tempo/grafana (Stage 16)
+loadtest/                        # module scrabble/loadtest: the pre-release stress harness (R2)
+backend/Dockerfile gateway/Dockerfile platform/telegram/Dockerfile loadtest/Dockerfile  # multi-stage distroless (Stage 16; loadtest R2)
+deploy/                          # docker-compose + caddy + otelcol/prometheus/tempo/grafana (+ cAdvisor/postgres_exporter, R2)
 ```
 
 ## Build & test