chore(deps): move test tooling out of the production image

[Ho1yShif]

Summary

requirements.txt shipped three test-only packages — pytest, pytest-asyncio, and httpx2 — which meant the production Render image baked them in for no reason. Dockerfile.render installs requirements.txt, and tests/ is excluded from the build context via .dockerignore, so this tooling could never run at runtime — it was pure image bloat. This moves the test deps into a new requirements-dev.txt (pins unchanged) so the shipped container installs runtime deps only, and updates the two places that actually run tests so nothing breaks.

This PR also carries two small fixes surfaced while getting CI green:

• ci.yml: Python 3.11 → 3.14 — the pinned deps target 3.14 (both Dockerfiles use python:3.14-slim; numpy==2.5.1 needs ≥3.12), so on 3.11 pip install failed before pytest could run. CI now tests the version that ships.
• README references docs/odysseus-browser.jpg — it was committed as a README screenshot but never linked, tripping test_no_orphan_images_in_docs; wired it in below the demo clip rather than deleting it.

Target branch

Intentionally targets main — for this template repo main is the default branch the Deploy to Render button forks and deploys, and it is currently ahead of dev. (The dev-target note in this template is inherited from upstream and doesn't match how this repo is curated.)

Linked Issue

N/A — issue tracking is disabled on this repository, so there is no issue to link. (This inherited template line does not apply here.)

Type of Change

• Refactor / cleanup (behaviour unchanged)
• CI / tooling / configuration

Checklist

• I searched open issues and PRs — this is not a duplicate.
• My changes are limited to the scope described above — no unrelated refactors or whitespace changes mixed in.
• I actually ran the app end-to-end. (Not claimed: I verified via pytest --co that the full suite still collects — 4655 tests — and confirmed no non-test runtime code imports pytest/httpx2, but did not boot the app for this dependency-only change.)

How to Test

1. pip install -r requirements.txt in a clean venv → confirm pytest is not installed (it's no longer a runtime dep).
2. pip install -r requirements.txt -r requirements-dev.txt → python -m pytest -q runs as before (this mirrors what CI now does).
3. Build the hosted image: docker build -f Dockerfile.render -t odysseus-render . → the image no longer contains pytest/pytest-asyncio/httpx2.

Visual / UI changes — REQUIRED if you touched anything that renders

No UI changes — dependency manifests, CI workflow, and CONTRIBUTING docs only.

[github-actions]

⚠️ PR description — action needed

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• Linked Issue — add a reference like Fixes #NNN, a bare #NNN, or a link to the issue.

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