Switch from fastembed to Python sentence-transformers for embedding

ort (ONNX Runtime) has no prebuilt binaries for aarch64-musl.
Use a Python subprocess with sentence-transformers instead:
- scripts/embed.py: reads JSON stdin, outputs embeddings
- kb.rs: calls Python script via tokio subprocess
- Dockerfile: install python3 + sentence-transformers

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

Dockerfile

@@ -8,13 +8,15 @@ RUN npm run build
 
 # Stage 2: Runtime
 FROM alpine:3.21
-RUN apk add --no-cache ca-certificates curl bash
+RUN apk add --no-cache ca-certificates curl bash python3 py3-pip
 RUN curl -LsSf https://astral.sh/uv/install.sh | sh
 ENV PATH="/root/.local/bin:$PATH"
+RUN pip3 install --break-system-packages sentence-transformers
 RUN mkdir -p /app/data/workspaces
 WORKDIR /app
 COPY target/aarch64-unknown-linux-musl/release/tori .
 COPY --from=frontend /app/web/dist ./web/dist/
+COPY scripts/embed.py ./scripts/
 COPY config.yaml .
 
 EXPOSE 3000