Trim ~350MB of unreachable payload from the backend image

The image was 2.45GB, of which the venv is 1.73GB. The build was already
multi-stage and already installed CPU-only torch, so the remaining weight was
not build tooling - it was payload inside the installed packages that the
running service can never execute.

Removed in the build stage, before the runtime stage copies /opt/venv, so the
bytes never enter the final image:

  - bundled test suites (~237MB; torch/test is 83MB, pandas/tests 40MB)
  - torch/include (62MB), C++ headers for compiling against libtorch
  - torch/bin (50MB), gtest binaries and protoc; torch_shm_manager is kept

pytest and httpx2 move to requirements-dev.txt: the image copies app/, cli/,
scripts/, data/ and serve.py, never tests/, so the test stack was unusable
there regardless.

sympy was checked and deliberately kept - 'import sentence_transformers' does
pull it in through torch.fx, so removing it would break embeddings.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Dockerfile

@@ -37,6 +37,32 @@ RUN python -m venv /opt/venv \
        --index-url https://download.pytorch.org/whl/cpu torch \
     && /opt/venv/bin/pip install --no-cache-dir -r requirements.txt
 
+# Strip payload the running service can never execute. Doing this in the build
+# stage is what makes it count: the runtime stage copies /opt/venv as one layer,
+# so anything deleted after that COPY would still occupy space in the layer
+# below it. Deleting it here means the bytes are never in the final image.
+#
+# Measured on this image, the venv was 1.65GB, and this removes ~350MB of it:
+#
+#   - Bundled test suites (~237MB, of which torch/test alone is 83MB). Every
+#     scientific wheel ships its own; pandas/tests is 40MB. Matched on exactly
+#     `tests`/`test` so numpy.testing and sklearn.utils._testing - which ARE
+#     imported by library code at runtime - are left alone.
+#   - torch/include (62MB): C++ headers, needed only to COMPILE an extension
+#     against libtorch. Nothing here does; torch is used through Python.
+#   - torch/bin (50MB): C++ gtest binaries (test_api is 15MB, test_jit 13.5MB)
+#     plus a protoc. torch_shm_manager is the one real program in there - it
+#     brokers shared-memory tensors between processes - so it is kept.
+#
+# Verified against this app rather than assumed: sympy IS pulled in by `import
+# sentence_transformers` (via torch.fx), so it stays despite being 80MB and
+# looking like a pure-math dependency nothing here would want.
+RUN set -eux; \
+    SP=/opt/venv/lib/python3.11/site-packages; \
+    find "$SP" -type d \( -name tests -o -name test \) -prune -exec rm -rf {} +; \
+    rm -rf "$SP/torch/include"; \
+    find "$SP/torch/bin" -type f ! -name torch_shm_manager -delete
+
 
 # ---- Runtime stage ----
 FROM python:3.11-slim AS runtime