EPIC: Vendor-Neutral Deployment, Multi-Database Support & Performance Validation

## Overview

This sub-EPIC decouples the deployment ecosystem from vendor-specific constraints, enables native MySQL database support alongside PostgreSQL, makes interactive demos platform-agnostic, and brings Artillery performance testing up to date.

## Goals

1. **Multi-Database Helm Support**:
- Rename database Secret key to `database-password` with backward-compatible fallback to `postgres-password`.
- Generalize `initContainers` to generic `wait-for-db` using dynamic host/port helpers.
- Add native MySQL support in `values.yaml` and `values.schema.json` with latest available versions
- Adapt network policy egress rules dynamically based on backend.
2. **Performance Test Modernization**:
- Bump Artillery test dependencies to latest versions.
- Fix broken test paths and token generation path resolution.
3. **Demo Platform Agnosticism**:
- Clean `demo/environment.yml` of Linux-specific build locks and add standard `requirements.txt` for macOS (Apple Silicon/Intel), Linux, and Windows.