fix(backend): use CPU-only PyTorch to shrink Docker image

.github/workflows/ci.yml

@@ -36,7 +36,7 @@ jobs:
           python-version: "3.11"
           cache: pip
           cache-dependency-path: |
-            apps/backend/requirements.txt
+            apps/backend/requirements-core.txt
             apps/backend/requirements-dev.txt
 
       - name: Install dependencies
@@ -62,13 +62,14 @@ jobs:
           python-version: "3.11"
           cache: pip
           cache-dependency-path: |
-            apps/backend/requirements.txt
+            apps/backend/requirements-core.txt
             apps/backend/requirements-dev.txt
 
       - name: Install backend dependencies
         run: |
           python -m pip install --upgrade pip
           python -m pip install torch==2.9.1 torchvision==0.24.1 --index-url https://download.pytorch.org/whl/cpu
+          python -m pip install -r apps/backend/requirements-core.txt
           python -m pip install -r apps/backend/requirements-dev.txt
 
       - name: Run backend tests

README.md

@@ -265,6 +265,7 @@ cd apps/backend
 python3 -m venv .venv
 source .venv/bin/activate
 python3 -m pip install --upgrade pip
+python3 -m pip install torch==2.9.1 torchvision==0.24.1 --index-url https://download.pytorch.org/whl/cpu
 python3 -m pip install -r requirements-dev.txt
 cp .env.example .env
 set -a

apps/backend/Dockerfile

@@ -14,8 +14,11 @@ RUN useradd --create-home --shell /bin/bash appuser
 
 WORKDIR ${APP_HOME}
 
-COPY requirements.txt ./requirements.txt
-RUN python -m pip install --upgrade pip && python -m pip install -r requirements.txt
+COPY requirements-core.txt ./requirements-core.txt
+RUN python -m pip install --upgrade pip && \
+    python -m pip install torch==2.9.1 torchvision==0.24.1 \
+      --index-url https://download.pytorch.org/whl/cpu && \
+    python -m pip install -r requirements-core.txt
 
 COPY app ./app
 COPY migrations ./migrations

apps/backend/README.md

@@ -45,6 +45,7 @@ app/
 tests/
 sql/manual/       optional manual seeds (calendar demo); not executed automatically
 requirements.txt
+requirements-core.txt
 requirements-dev.txt
 Dockerfile
 ```
@@ -55,6 +56,7 @@ Dockerfile
 python3 -m venv .venv
 source .venv/bin/activate
 python3 -m pip install --upgrade pip
+python3 -m pip install torch==2.9.1 torchvision==0.24.1 --index-url https://download.pytorch.org/whl/cpu
 python3 -m pip install -r requirements-dev.txt
 cp .env.example .env
 set -a
@@ -119,6 +121,8 @@ Important settings:
 
 ## Docker
 
+The production image installs CPU-only PyTorch wheels (not the default CUDA build from PyPI), keeping the image around **2–3 GB** instead of ~9 GB. Inference remains CPU-only (`DEVICE=cpu`).
+
 ### Build
 
 ```bash

apps/backend/requirements-core.txt

@@ -0,0 +1,17 @@
+# Install CPU torch first — see Dockerfile and apps/backend/README.md
+# pip install torch==2.9.1 torchvision==0.24.1 --index-url https://download.pytorch.org/whl/cpu
+fastapi
+python-multipart
+Pillow
+requests
+sqlalchemy
+alembic
+psycopg[binary]
+boto3
+numpy
+joblib
+scikit-learn
+huggingface_hub
+transformers
+uvicorn[standard]
+prometheus-fastapi-instrumentator

apps/backend/requirements-dev.txt

@@ -1,5 +1,4 @@
--r requirements.txt
+-r requirements-core.txt
 
 httpx
 pytest
-

apps/backend/requirements.txt

@@ -1,18 +1,4 @@
-fastapi
-python-multipart
-Pillow
-requests
-sqlalchemy
-alembic
-psycopg[binary]
-boto3
-numpy
-joblib
-scikit-learn
-huggingface_hub
-transformers
-torch==2.9.1
-torchvision==0.24.1
-uvicorn[standard]
-prometheus-fastapi-instrumentator
-
+# PyTorch is installed separately from the CPU wheel index.
+# Docker: see Dockerfile
+# Local / CI: pip install torch==2.9.1 torchvision==0.24.1 --index-url https://download.pytorch.org/whl/cpu
+-r requirements-core.txt