chore(env): standardize backend python on 3.12

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

.ai/SESSION_LOG.md

@@ -12,6 +12,14 @@
 
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+## 2026-05-07 11:30 EDT — Codex — Standardize backend Python on 3.12
+
+- Standardized repo declarations around Python 3.12: added `.python-version` pinned to 3.12.13, updated stale Python 3.11 docs, and added explicit Python 3.12 setup steps to Gitea CI. GitHub CI was already updated to Python 3.12 by the user.
+- Installed pyenv Python 3.12.13 and created `backend/venv` from that interpreter. Installed `backend/requirements-dev.txt` into the venv.
+- Verified native `python --version` and venv `python --version` both report 3.12.13. Verified native `pytest 8.4.2` and `alembic 1.18.3` with explicit safe test env vars; plain pytest import still depends on local `.env` values being valid.
+- Rebuilt and restarted the dev backend container with `docker compose -f docker-compose.dev.yml build backend` and `up -d backend`; confirmed `docker exec resolutionflow_backend python --version` reports 3.12.13.
+- Files touched: `.python-version`, `.gitea/workflows/ci.yml`, `.github/workflows/ci.yml`, `README.md`, `DEV-ENV.md`, `.ai/PROJECT_CONTEXT.md`, `.ai/DECISIONS.md`, `.ai/HANDOFF.md`, `.ai/SESSION_LOG.md`.
+
 ## 2026-05-07 11:14 EDT — Codex — Recheck native Python availability
 
 - Re-ran the startup ritual and checked the host Python state after the user reported fixing the missing native Python issue.