fix: harden hysteria auth boundaries

[smileoniks-ctrl]

Summary

This PR hardens the public Hysteria authentication and subscription boundaries.

• Rejects malformed /api/auth payloads unless auth is exactly userId:password with both parts non-empty.
• Always validates the supplied password against the generated Hysteria password first, then the legacy DB password fallback.
• Removes public userId fallback from /api/files/:token and /api/info/:token; only subscriptionToken is accepted.
• Removes panel UI and wizard fallbacks that generated subscription links with userId.
• Adds startup backfill for existing users without subscriptionToken.
• Adds a dedicated /api/auth rate limiter using rateLimit.authPerSecond.

Why

Previously, strict auth could be bypassed by sending a bare userId or malformed auth payload. Public subscription endpoints also accepted userId as a token fallback, exposing predictable subscription URLs.

Compatibility

Generated Hysteria links still use:

hysteria2://userId:password@...