R2: drop ./loadtest from the backend/gateway/telegram image builds

Adding the loadtest module to go.work (use ./loadtest + the scrabble/gateway
replace it needs) broke the other services' Docker builds: their reduced
workspace still referenced ./loadtest (not in their build context), failing with
'cannot load module loadtest: open loadtest/go.mod: no such file or directory'.
Each service Dockerfile now also -dropuse=./loadtest; backend and telegram (which
do not COPY ./gateway) additionally -dropreplace the loadtest-only scrabble/gateway
replace. Verified by building all three images plus loadtest locally.

platform/telegram/Dockerfile

@@ -1,8 +1,9 @@
 # Telegram connector image.
 #
 # The connector imports only the shared scrabble/pkg module, so the build drops the
-# other workspace modules (backend, gateway) and the scrabble-solver replace from a
-# copy of go.work: it needs neither their sources nor the solver sibling checkout.
+# other workspace modules (backend, gateway, loadtest), the loadtest-only
+# scrabble/gateway replace and the scrabble-solver replace from a copy of go.work: it
+# needs neither their sources nor the solver sibling checkout.
 # Build from the repository ROOT so go.work, pkg/ and platform/telegram/ are all in
 # the context (see deploy/docker-compose.yml, which sets context: ../../..).
 FROM golang:1.26.3-alpine AS build
@@ -13,7 +14,7 @@ COPY pkg ./pkg
 COPY platform/telegram ./platform/telegram
 
 # Reduce the workspace to what the connector needs: only pkg + platform/telegram.
-RUN go work edit -dropuse=./backend -dropuse=./gateway -dropreplace=scrabble-solver
+RUN go work edit -dropuse=./backend -dropuse=./gateway -dropuse=./loadtest -dropreplace=scrabble/gateway@v0.0.0 -dropreplace=scrabble-solver
 
 RUN CGO_ENABLED=0 GOOS=linux go build -trimpath -o /out/telegram ./platform/telegram/cmd/telegram