chore(security): add OSS Scanner build image and threat model

Anthropic's OSS Scanner builds .oss-scanner/Dockerfile from this repository
and audits the image offline. The image keeps the source, the Go module and
npm caches, the debug panel binary, Xray-core and the sidecars, so tests,
the xray gRPC e2e suite, the master+node e2e and a live panel all run with
the network disabled. threat_model.md lists the trust boundaries, entry
points and the severity rubric the scanner should report against.

.oss-scanner/Dockerfile

@@ -0,0 +1,59 @@
+# Build environment for Anthropic's OSS Scanner (https://github.com/anthropics/oss-scanner).
+# The scanner builds this with the repository as the context, then audits the image offline:
+# everything the build, the tests and a reproducer need must be fetched here.
+
+FROM node:26-trixie-slim AS node
+
+FROM golang:1.27.2-trixie
+
+ENV DEBIAN_FRONTEND=noninteractive \
+    CGO_ENABLED=1 \
+    CGO_CFLAGS="-D_LARGEFILE64_SOURCE" \
+    GOTOOLCHAIN=local
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+      build-essential ca-certificates curl git jq procps sqlite3 unzip \
+    && rm -rf /var/lib/apt/lists/*
+
+# Node 26 is required: `make gen` imports .ts files directly and needs its type stripping.
+COPY --from=node /usr/local/bin/node /usr/local/bin/node
+COPY --from=node /usr/local/lib/node_modules /usr/local/lib/node_modules
+RUN ln -s ../lib/node_modules/npm/bin/npm-cli.js /usr/local/bin/npm \
+    && ln -s ../lib/node_modules/npm/bin/npx-cli.js /usr/local/bin/npx \
+    && node --version && npm --version
+
+# scanner contract: the checkout lives inside the image, at /src
+COPY . /src
+WORKDIR /src
+
+RUN go mod download && go mod verify
+
+# The Go binary embeds the Vite build from internal/web/dist, so the frontend goes first.
+# HUSKY=0 skips the git-hook install, which needs a .git directory the build context may lack.
+RUN cd frontend && HUSKY=0 npm ci && npm run build
+
+# Debug info and no inlining, so stack traces and debuggers map straight to the source.
+RUN go build -gcflags=all="-N -l" -o /usr/local/bin/x-ui . \
+    && ln -s /usr/local/bin/x-ui /src/x-ui
+
+# Xray-core, the mtg-multi MTProto sidecar and the geo databases, exactly as the release image ships them.
+RUN ./DockerInit.sh "$(dpkg --print-architecture)" \
+    && ln -s "/src/build/bin/xray-linux-$(dpkg --print-architecture)" /usr/local/bin/xray \
+    && xray version
+
+# Turns on the xray gRPC end-to-end tests (internal/xray, internal/amneziawgnet) against the real core.
+ENV XRAY_E2E_BINARY=/usr/local/bin/xray
+
+# Baseline tests: also warms the build cache. A failure is logged, never fatal.
+RUN go test -count=1 -timeout 30m ./... > /src/.oss-scanner-go-test.log 2>&1 \
+    || echo "WARNING: go test reported failures, see /src/.oss-scanner-go-test.log"
+RUN cd frontend && npx vitest run --project unit --project components > /src/.oss-scanner-vitest.log 2>&1 \
+    || echo "WARNING: vitest reported failures, see /src/.oss-scanner-vitest.log"
+
+# Reproducers and patches must build with the network gone.
+ENV GOPROXY=off \
+    GOSUMDB=off \
+    GOFLAGS=-mod=readonly \
+    npm_config_offline=true
+
+CMD ["bash"]