docs: add technical specification for CandyConnect VPN improvements

[malliancea]

Motivation

• Formalize and capture requirements to fix OpenVPN config generation and download behavior in the web panel.
• Define a user-driven WireGuard flow where QR codes are generated on demand from the client profile instead of auto-assigning private keys.
• Remove the DNSTT connection type and specify replacement with Amnezia including install and deployment changes.

Description

• Added a new technical specification document at docs/ТЗ_CandyConnect_VPN.md that details goals, problems, scope, API/UX contracts, and acceptance criteria for OpenVPN, WireGuard, and Amnezia changes.
• Specified functional and non-functional requirements for OpenVPN config generation and clickable download behavior, and WireGuard QR generation and idempotency.
• Specified required changes to remove DNSTT and add AMNEZIA across backend enums, frontend selectors, install scripts, and health checks.
• Included testing scope, integration/UI/e2e and smoke test plans, migration risks, and expected outcomes for the delivery.

Testing

• Created and reviewed the document file docs/ТЗ_CandyConnect_VPN.md and committed it to the branch using git commit, which succeeded.
• Verified file contents with nl -ba docs/ТЗ_CandyConnect_VPN.md | sed -n '1,240p', which produced the document preview successfully.
• Verified repository state and commit details with git status --short and git show --stat --oneline HEAD, both of which succeeded.

---

Codex Task