chore: split dev/test dependencies into requirements-dev.txt

[man4ish]

What

PR #9 pinned requirements.txt to production dependencies only. Since then, running the test suite has required installing dev tooling manually (tribal knowledge) — nothing declared it. This PR adds requirements-dev.txt for those packages.

Packages moved into requirements-dev.txt (pinned to currently-installed working versions)

Package	Version	Why it's needed
pytest	9.0.2	test runner
pytest-asyncio	1.3.0	runs the @pytest.mark.asyncio tests in tests/test_main_api.py
pytest-cov	7.0.0	coverage tooling (.coveragerc already in repo)
httpx	0.28.1	required by starlette.testclient.TestClient, used in test_main_api.py, test_auth.py, test_rag_routes.py
ruff	0.16.1	linting, previously installed ad-hoc in CI (pip install ruff pytest)

None of these are imported by application code — requirements.txt alone is sufficient to run the app.

Verification (done from scratch, two separate fresh venvs)

venv 1 — requirements.txt only (production-only, no dev deps):

• pip install -r requirements.txt succeeds
• api.main.app imports and boots via uvicorn
• GET /health returns 200 {"status":"ok", ...}
• confirms requirements.txt alone is sufficient to run the app, not to test it

venv 2 — requirements.txt + requirements-dev.txt together:

• both install cleanly
• pytest -q → 199 passed, no missing-dependency errors
• pytest-asyncio correctly registers in strict mode and the two async tests exercise real coroutines (not silently skipped)

CI config diff

       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
           if [ -f pyproject.toml ]; then
             pip install -e ".[dev]" 2>/dev/null || pip install -e .
           elif [ -f requirements.txt ]; then
-            pip install -r requirements.txt
+            pip install -r requirements.txt -r requirements-dev.txt
           fi
-          pip install ruff pytest

CI's install step now matches what's actually needed to run pytest — no more ad-hoc pip install ruff pytest disconnected from any tracked file.

Also updated

README.md now documents the dev/test install path (pip install -r requirements.txt -r requirements-dev.txt) separately from the production install command.

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