fix(setup): pin dev venv to Python 3.12

backend/pyproject.toml

@@ -1,7 +1,12 @@
 [project]
 name = "voicebox-backend"
 version = "0.2.3"
-requires-python = ">=3.12"
+# This is the one place the Python pin is explained; the justfile and docs point here
+# rather than restating it. The upper bound is real, not defensive: requirements.txt
+# currently pulls in kokoro and misaki (which declare requires-python <3.13) plus
+# numpy<2 and numba<0.61 (no 3.13 wheels), so `pip install -r requirements.txt` fails
+# outright on a later interpreter. Recheck those four before raising it.
+requires-python = ">=3.12,<3.13"
 
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