fix: use CPU-only PyTorch and run init_db before alembic migrations

PyTorch default install pulls ~7GB of CUDA libs, exceeding disk on small
VMs. Switching to CPU-only saves ~6GB. Also run create_all before alembic
so migrations find existing tables on a fresh Postgres.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/Dockerfile

@@ -24,7 +24,11 @@ WORKDIR /app
 
 # Copy requirements first for better caching
 COPY requirements.txt .
-RUN pip install --no-cache-dir -r requirements.txt
+# Install PyTorch CPU-only FIRST so open-clip-torch doesn't pull the full
+# CUDA build (~7 GB).  CPU inference is all we need — the heavy lifting
+# happens through ONNX Runtime.
+RUN pip install --no-cache-dir torch torchvision --index-url https://download.pytorch.org/whl/cpu \
+    && pip install --no-cache-dir -r requirements.txt
 
 # Copy application code
 COPY . .