feat(backends): darwin/Metal build for the privacy-filter backend (timeboxed try)

The privacy-filter.cpp engine is already Metal-capable on Apple Silicon: it pulls
ggml and never forces GGML_METAL=OFF, and ggml defaults Metal ON on Apple, so a
plain Darwin build is Metal-enabled. grpc++/protobuf resolve from Homebrew via
find_package(... CONFIG). It just had no darwin build path - the existing
package.sh and run.sh are Linux-only and there was no make target / workflow step.

Adds the bespoke darwin path, modeled on the ds4 one:
- scripts/build/privacy-filter-darwin.sh: native make grpc-server, otool -L dylib
  bundling, create-oci-image (no Linux package.sh).
- Makefile: backends/privacy-filter-darwin target (+ .NOTPARALLEL).
- .github/workflows/backend_build_darwin.yml: gated build step for privacy-filter.
- scripts/changed-backends.js: inferBackendPathDarwin special-case -> backend/cpp.
- .github/backend-matrix.yml: includeDarwin entry (lang go, like ds4/llama-cpp).
- backend/index.yaml: metal: capability + metal-privacy-filter(-development) entries.
- backend/cpp/privacy-filter/run.sh: DYLD_LIBRARY_PATH branch on Darwin.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Assisted-by: Claude:opus-4.8 [Claude Code]

.github/workflows/backend_build_darwin.yml

@@ -228,8 +228,17 @@ jobs:
         run: |
           make backends/ds4-darwin
 
+      # privacy-filter is a C++/ggml backend like ds4 - a single grpc-server with
+      # otool dylib bundling - so it gets its own bespoke darwin script rather than
+      # the generic build-darwin-go-backend path.
+      - name: Build privacy-filter backend (Darwin Metal)
+        if: inputs.backend == 'privacy-filter'
+        run: |
+          make protogen-go
+          make backends/privacy-filter-darwin
+
       - name: Build ${{ inputs.backend }}-darwin
-        if: inputs.backend != 'llama-cpp' && inputs.backend != 'ds4'
+        if: inputs.backend != 'llama-cpp' && inputs.backend != 'ds4' && inputs.backend != 'privacy-filter'
         run: |
           make protogen-go
           BACKEND=${{ inputs.backend }} BUILD_TYPE=${{ inputs.build-type }} USE_PIP=${{ inputs.use-pip }} make build-darwin-${{ inputs.lang }}-backend