Add cv2/tqdm to base; implement per-project Python venvs (Design B, Phase 1)

Base image: add opencv-python-headless (cv2) and tqdm to the bundled
scientific stack, and python3-venv (needed to build per-project venvs).

Per-project dependencies: a project's requirements.txt is now installed into a
venv cached by its sha256 (python3 -m venv --system-site-packages, so the
bundled stack stays visible and only extra packages are installed); QuartoRunner
points Quarto at it via QUARTO_PYTHON. A per-hash flock serialises concurrent
builds; pip output is merged into output.log; on failure the render falls back
to the base interpreter. Venvs live under PYTHON_VENVS_DIR
(default /var/lib/overleaf/data/python-venvs).

Gating: PythonVenvGate.userCanInstallPython restricts installs to the project
owner + invited collaborators (ignorePublicAccess excludes anonymous/link
users), threaded to CLSI as allowPythonInstall on the editor compile,
presentation export, and publish paths. Behind OVERLEAF_ENABLE_PROJECT_PYTHON_VENV
(enabled in the deployment). Design doc updated; Phase 2 (egress policy) and
Phase 3 (venv eviction) remain.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/deploy-verso.yml

@@ -290,6 +290,12 @@ jobs:
                       # need OVERLEAF_ALLOW_PUBLIC_ACCESS above.
                       - name: OVERLEAF_ALLOW_ANONYMOUS_READ_AND_WRITE_SHARING
                         value: "true"
+                      # Let Quarto Python cells use a project's requirements.txt:
+                      # the compiler installs it into a cached venv. Gated to the
+                      # project owner + invited collaborators (never anonymous /
+                      # link-sharing users).
+                      - name: OVERLEAF_ENABLE_PROJECT_PYTHON_VENV
+                        value: "true"
           ---
           apiVersion: v1
           kind: Service