build: pin one Python version across the app, the images and the gate

0.93.2 shipped a week's worth of confusion about which Node this repo
runs on; the Python side had the identical shape and nobody had looked.
Prod (Dockerfile), the test image (Dockerfile.test) and CI (check.yml)
each spelled 3.12 as an independent literal, there was no
`.python-version`, no `requires-python`, and ruff had no target-version
so it judged our syntax against some other interpreter. The dev machine
had no 3.12 installed at all — only 3.13 and 3.14 — so every ad-hoc host
script ran two majors ahead of production.

`.python-version` is now the single source of truth, and the contract is
held by tests rather than by prose. It is split in two because no single
runner can see every link: the pytest suite runs inside the test image,
which mounts backend/ alone and cannot read the repo-root Dockerfile or
the workflow.

  test_python_engine_contract.py : running interpreter == pyrightconfig
                                   == ruff.toml
  pythonEngine.contract.test.ts  : .python-version == pyrightconfig ==
                                   ruff.toml == Dockerfile ==
                                   Dockerfile.test == check.yml

Together they close the loop, and the backend half carries the assertion
that pays — that the interpreter running the suite is the pinned one. A
green suite on another minor does not mean the shipped image is green;
it means they have drifted and nobody has noticed yet.

backend/ruff.toml

@@ -7,6 +7,10 @@
 # the repo wrap at the same width, and it's the width `ruff format` enforces (gated via
 # `ruff format --check` alongside `ruff check`).
 line-length = 100
+# The interpreter this code actually runs on, so ruff judges syntax and upgrade hints against the
+# right one. Left unset it falls back to a default that has nothing to do with us. `/.python-version`
+# is the single source of truth; tests/test_python_engine_contract.py holds this to it.
+target-version = "py312"
 
 [lint]
 # Beyond the default set, the rules that hold a LOG LINE to what E22.5 makes of it:

backend/tests/test_python_engine_contract.py

@@ -0,0 +1,66 @@
+"""The Python major is a contract, not a coincidence — the backend half (E27 S3).
+
+`0.93.2` was released to fix exactly this bug on the JavaScript side: four machines built or checked
+the app and none of them agreed on which Node they ran, the server gate went red for 73 of its last
+90 runs, and nothing told anyone. The Python side had the same shape and nobody had looked: prod
+(`Dockerfile`), the test image (`Dockerfile.test`) and CI (`check.yml`) each spelled **3.12** as an
+independent literal, there was no `.python-version`, no `requires-python`, and ruff had no
+`target-version` — while the dev machine had no 3.12 installed at all, only 3.13 and 3.14.
+
+**The contract is a chain, and it is split across two test files because no single runner can see
+every link.** This suite runs inside the test image, which mounts `backend/` and nothing else, so it
+cannot read the repo-root Dockerfile or the workflow. It therefore holds the link it CAN see, and the
+one that matters most here:
+
+    the interpreter actually running  ==  backend/pyrightconfig.json  ==  backend/ruff.toml
+
+and `frontend/src/lib/pythonEngine.contract.test.ts` — which runs on the host, with the whole tree —
+holds the rest of the chain:
+
+    /.python-version  ==  pyrightconfig.json  ==  ruff.toml  ==  Dockerfile  ==  Dockerfile.test  ==  check.yml
+
+Together they close the loop. To move the version: edit `/.python-version`, then follow the failures
+in both files. Every place they name is a place that would otherwise have been forgotten.
+"""
+
+import re
+import sys
+from pathlib import Path
+
+BACKEND = Path(__file__).resolve().parents[1]
+
+
+def _version_in(path: Path, pattern: str) -> tuple[int, int]:
+    """The (major, minor) the given file declares. A rename or a moved line lands here — re-point
+    the test, the contract did not go away with it."""
+    text = path.read_text(encoding="utf-8")
+    match = re.search(pattern, text, re.MULTILINE)
+    assert match is not None, f"{pattern!r} not found in {path.name}"
+    return int(match.group(1)), int(match.group(2))
+
+
+# pyrightconfig.json is JSONC (it carries comments), so it is read by regex rather than json.loads.
+PINNED = _version_in(BACKEND / "pyrightconfig.json", r'"pythonVersion"\s*:\s*"(\d+)\.(\d+)"')
+
+
+def test_pins_a_plausible_python_version():
+    """Guards against a typo agreeing with nothing at all."""
+    assert PINNED >= (3, 12), PINNED
+
+
+def test_this_interpreter_is_the_pinned_one():
+    """The assertion that pays. A green suite on another minor does not mean the shipped image is
+    green — it means the suite and the image have drifted and nobody noticed yet."""
+    running = sys.version_info[:2]
+    assert running == PINNED, (
+        f"these tests are running on Python {running[0]}.{running[1]}, but the app is pinned to "
+        f"{PINNED[0]}.{PINNED[1]} (backend/pyrightconfig.json, /.python-version). The suite runs in "
+        f"the test image, so this failing means backend/Dockerfile.test has drifted from the pin."
+    )
+
+
+def test_ruff_judges_the_pinned_version():
+    """Unset, ruff falls back to a default that has nothing to do with us, and its syntax and
+    upgrade advice are then aimed at some other interpreter."""
+    major, minor = _version_in(BACKEND / "ruff.toml", r'^target-version\s*=\s*"py(\d)(\d+)"')
+    assert (major, minor) == PINNED