feat(deploy): Docker + Linux server deployment

Multi-stage Dockerfile (builder → export → runtime) producing a 149 MB
Ubuntu 24.04 image, verified booting end-to-end on Docker Desktop.  vcpkg
fetched via shallow git fetch at the pinned baseline, release-only overlay
triplets (x64-linux, arm64-linux) to halve intermediate disk usage, and
buildtrees deleted within the RUN layer so they never land in the image or
the BuildKit cache.  Binary cache mount (VCPKG_BINARY_SOURCES) makes
subsequent rebuilds restore pre-built packages instead of recompiling.

Also adds:
- docker-compose.yml for one-command local deploy
- .dockerignore (excludes clients/, build/, .git/)
- .github/workflows/build-linux.yml — CI cross-build for amd64 + arm64
  with downloadable artifacts (primary path for building from Windows)
- scripts/build-linux-binaries.sh — local Docker binary extraction fallback
- deploy/linux/voicecat.service — hardened systemd unit for bare-metal
- cmake/voicecat-toolchain.cmake now auto-wires VCPKG_OVERLAY_TRIPLETS

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/build-linux.yml

@@ -0,0 +1,87 @@
+name: Build Linux Binaries
+
+# Builds stripped voicecat-server + voicecat-admin for linux/amd64 and linux/arm64.
+# Run manually from the Actions tab, or on any push to main.
+# Artifacts are downloadable from the workflow run for ~90 days.
+
+on:
+  workflow_dispatch:        # manual trigger from the Actions tab
+  push:
+    branches: [main]
+    paths:
+      - 'core/**'
+      - 'server/**'
+      - 'tools/**'
+      - 'cmake/**'
+      - 'CMakeLists.txt'
+      - 'CMakePresets.json'
+      - 'vcpkg.json'
+      - 'Dockerfile'
+      - '.github/workflows/build-linux.yml'
+
+jobs:
+  build:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - arch: amd64
+            runner: ubuntu-24.04
+            vcpkg_triplet: x64-linux
+          - arch: arm64
+            runner: ubuntu-24.04-arm
+            vcpkg_triplet: arm64-linux
+
+    name: linux/${{ matrix.arch }}
+    runs-on: ${{ matrix.runner }}
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Cache vcpkg packages
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cache/vcpkg
+            /usr/local/share/vcpkg/buildtrees
+          key: vcpkg-${{ matrix.vcpkg_triplet }}-${{ hashFiles('vcpkg.json') }}
+          restore-keys: |
+            vcpkg-${{ matrix.vcpkg_triplet }}-
+
+      - name: Install build dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y --no-install-recommends \
+            build-essential cmake ninja-build git curl zip unzip tar \
+            pkg-config autoconf autoconf-archive automake libtool nasm python3
+
+      - name: Set up vcpkg
+        run: |
+          VCPKG_COMMIT=$(jq -r '."builtin-baseline"' vcpkg.json)
+          git init /tmp/vcpkg
+          git -C /tmp/vcpkg remote add origin https://github.com/microsoft/vcpkg.git
+          git -C /tmp/vcpkg fetch --depth=1 origin "$VCPKG_COMMIT"
+          git -C /tmp/vcpkg checkout FETCH_HEAD
+          /tmp/vcpkg/bootstrap-vcpkg.sh -disableMetrics
+          echo "VCPKG_ROOT=/tmp/vcpkg" >> "$GITHUB_ENV"
+          echo "VCPKG_DISABLE_METRICS=1" >> "$GITHUB_ENV"
+
+      - name: Build (server-release preset)
+        run: |
+          cmake --preset server-release
+          cmake --build --preset server-release
+
+      - name: Collect binaries
+        run: |
+          mkdir -p dist
+          cp build/server-release/bin/voicecat-server dist/
+          cp build/server-release/bin/voicecat-admin  dist/
+          file dist/voicecat-server dist/voicecat-admin
+          ls -lh dist/
+
+      - name: Upload binaries
+        uses: actions/upload-artifact@v4
+        with:
+          name: voicecat-linux-${{ matrix.arch }}
+          path: dist/
+          retention-days: 90