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.

.gitea/workflows/ci.yaml

@@ -64,7 +64,7 @@ jobs:
           if [ "$files" != "__DIFF_FAILED__" ]; then
             echo "changed files:"; echo "$files"
             go=false; ui=false
-            if echo "$files" | grep -qE '^(backend/|pkg/|gateway/|platform/|go\.work)'; then go=true; fi
+            if echo "$files" | grep -qE '^(backend/|pkg/|gateway/|platform/|loadtest/|go\.work)'; then go=true; fi
             if echo "$files" | grep -qE '^ui/'; then ui=true; fi
             # A workflow or deploy change re-runs everything as a safety net.
             if echo "$files" | grep -qE '^(\.gitea/workflows/|deploy/)'; then go=true; ui=true; fi
@@ -112,15 +112,15 @@ jobs:
           fi
 
       - name: vet
-        run: go vet ./backend/... ./pkg/... ./gateway/... ./platform/telegram/...
+        run: go vet ./backend/... ./pkg/... ./gateway/... ./platform/telegram/... ./loadtest/...
 
       - name: build
-        run: go build ./backend/... ./pkg/... ./gateway/... ./platform/telegram/...
+        run: go build ./backend/... ./pkg/... ./gateway/... ./platform/telegram/... ./loadtest/...
 
       - name: test
         env:
           BACKEND_DICT_DIR: ${{ github.workspace }}/dawg
-        run: go test -count=1 ./backend/... ./pkg/... ./gateway/... ./platform/telegram/...
+        run: go test -count=1 ./backend/... ./pkg/... ./gateway/... ./platform/telegram/... ./loadtest/...
 
   integration:
     needs: changes