chore(env): standardize backend python on 3.12

Co-Authored-By: Codex <noreply@openai.com>

.ai/HANDOFF.md

@@ -2,7 +2,7 @@
 
 # HANDOFF.md
 
-**Last updated:** 2026-05-07 (native Python environment rechecked; cutover ops still pending)
+**Last updated:** 2026-05-07 (Python 3.12 standardization applied; cutover ops still pending)
 
 **Active task:** None mid-flight. Branch `feat/self-serve-signup-phase-2` (HEAD `502c0a4`) is ready to PR.
 
@@ -63,7 +63,7 @@ New env vars (Vite ARG/ENV in Dockerfile + `.env.example`): `VITE_SELF_SERVE_ENA
 
 ## Environment notes (carry-forward)
 
-- Code-server LXC now has native `python`/`python3` via pyenv at Python 3.12.10, plus `pip` 25.0.1. There is no native `python3.11`, no repo virtualenv, and the native environment has only pip installed (`python -m pytest --version` fails with `No module named pytest`). Backend remains documented for Python 3.11, so use Docker for authoritative backend validation until a 3.11 env is installed.
+- Code-server LXC now standardizes on Python 3.12. `.python-version` pins 3.12.13 to match the Docker image; native backend work should use `backend/venv` built from that interpreter. `pytest --version` and `alembic --version` pass natively when `DEBUG=true SECRET_KEY=ci-test-secret-key-not-for-production` are supplied to avoid local `.env` validation failures.
 - Pytest: `docker exec resolutionflow_backend pytest tests/<file> -v --override-ini="addopts="`.
 - Vitest: `docker exec -w /app resolutionflow_frontend npm test -- <path> --run`.
 - TS build: `docker exec -w /app resolutionflow_frontend npx tsc -b`.