feat(backend): add chat service with evidence gating and citation validation

backend/Dockerfile

@@ -4,7 +4,8 @@ ENV PYTHONUNBUFFERED=1 \
     PYTHONDONTWRITEBYTECODE=1 \
     PIP_NO_CACHE_DIR=1 \
     OMP_NUM_THREADS=2 \
-    TOKENIZERS_PARALLELISM=false
+    TOKENIZERS_PARALLELISM=false \
+    TORCH_HOME=/data/models
 
 # tesseract-ocr + pacote de idioma português + libs de imagem do MuPDF
 RUN apt-get update && apt-get install -y --no-install-recommends \
@@ -16,13 +17,18 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 
 WORKDIR /app
 
+# PyTorch só com CPU: o wheel padrão traz ~4 GB de bibliotecas CUDA, inúteis aqui.
+RUN pip install --no-cache-dir \
+        --index-url https://download.pytorch.org/whl/cpu \
+        "torch==2.5.1"
+
 COPY requirements.txt requirements-dev.txt ./
 RUN pip install --no-cache-dir -r requirements-dev.txt
 
 COPY app ./app
 COPY tests ./tests
 
-RUN mkdir -p /data/vector_db /data/pdfs
+RUN mkdir -p /data/vector_db /data/pdfs /data/models
 
 EXPOSE 8000