refactor(environment): update virtualenv path to /opt/venv in Docker setup

- Adjusted `docker-compose.dev.yml` to reflect the new venv location.
- Modified entrypoint script and Dockerfile to reference `/opt/venv` for isolated dependencies.
- Improved bind mount setup to prevent venv overwrites during development.

backend/entrypoint.sh

@@ -1,11 +1,11 @@
 #!/bin/bash
 set -e
 
-# Ensure the project's virtualenv binaries are on PATH so commands like
-# 'uvicorn' work even when not prefixed by 'uv run'. This matches how uv
-# installs the env into /app/.venv in our containers.
-if [ -d "/app/.venv/bin" ]; then
-  export PATH="/app/.venv/bin:$PATH"
+# Ensure the virtualenv binaries are on PATH. Dependencies are installed
+# to /opt/venv (not /app/.venv) to survive bind mounts in development.
+if [ -d "/opt/venv/bin" ]; then
+  export PATH="/opt/venv/bin:$PATH"
+  export VIRTUAL_ENV="/opt/venv"
 fi
 
 # Only the backend service should run migrations and init_db