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>

requirements-dev.txt

@@ -0,0 +1,14 @@
+# Test-only dependencies, kept out of requirements.txt so the Docker image does
+# not ship them - the container has no tests/ directory to run anyway (the
+# Dockerfile copies app/, cli/, scripts/, data/ and serve.py, nothing else).
+#
+# For development, install both files:
+#     pip install -r requirements.txt -r requirements-dev.txt
+
+pytest>=8.3.3
+
+# Starlette's TestClient deprecates the httpx 0.x backend and emits a
+# StarletteDeprecationWarning without this. pytest.ini turns warnings into
+# errors, so it is a hard requirement of the suite, not a nicety. Test-only:
+# the application itself uses the `httpx` pinned in requirements.txt.
+httpx2>=2.10.0