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>

README.md

@@ -32,7 +32,7 @@ It does **not** start Postgres or Ollama for you - it reports them via
 cd backend
 python3 -m venv .venv
 source .venv/bin/activate          # Windows: .venv\Scripts\activate
-pip install -r requirements.txt
+pip install -r requirements.txt -r requirements-dev.txt   # -dev is pytest only
 
 cp .env.example .env               # then edit DB_PASSWORD etc.
 
@@ -198,7 +198,8 @@ backend/
 ├── cli/ingest_brand.py           # CLI: ingest one brand end-to-end
 ├── scripts/seed_sample_data.py   # load bundled sample catalogs (no LLM needed)
 ├── data/seed_catalogs/*.json     # bundled sample catalogs (Parle, Cadbury, ...)
-└── requirements.txt
+├── requirements.txt
+└── requirements-dev.txt
 ```
 
 ## Running tests