chore(env): standardize backend python on 3.12

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

DEV-ENV.md

@@ -108,7 +108,7 @@ Run these in order. Stop at the first failure and investigate.
 # Ubuntu / Debian
 sudo apt update && sudo apt install -y \
   git curl build-essential \
-  python3.11 python3.11-venv python3-pip \
+  python3.12 python3.12-venv python3-pip \
   postgresql-client   # not the server — only if running Postgres natively
 
 # Node 20 via nvm (survives container rebuilds if stored in a volume)
@@ -236,7 +236,7 @@ REPO_ROOT=/absolute/path/to/resolutionflow
 
 ```bash
 cd backend
-python3.11 -m venv venv
+python3.12 -m venv venv
 source venv/bin/activate
 pip install -r requirements.txt