docs: refresh contributor docs and drop root requirements.txt

CONTRIBUTING.md caught up with reality: ruff not Black, Python 3.12,
sh.voicebox.app, the routes/services/backends layout, the actual
pytest + CI story, and a working autoupdater link. SECURITY.md now
says 0.5.x and describes what CI actually enforces. The root
requirements.txt was vestigial (unpinned, included torchvision,
referenced by nothing) — deleted, with two stale doc references
repointed at backend/requirements.txt. backend/pyproject.toml is now
covered by bumpversion; its version was stuck at 0.2.3 and is synced
to 0.5.0.

docs/plans/DOCKER_DEPLOYMENT.md

@@ -90,7 +90,7 @@ RUN apt-get update && apt-get install -y \
 
 # Copy application
 COPY backend/ /app/backend/
-COPY requirements.txt /app/
+COPY backend/requirements.txt /app/
 
 # Install Python dependencies
 RUN pip install --no-cache-dir -r requirements.txt