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.txt

@@ -76,9 +76,6 @@ scipy>=1.13.1
 joblib>=1.4.2
 
 # --- Dev/test tooling ---
-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 above.
-httpx2>=2.10.0
+# Moved to requirements-dev.txt so the Docker image does not carry the test
+# stack it can never run. To work on this project, install both:
+#     pip install -r requirements.txt -r requirements-dev.txt