Conider making email optional for user accounts

[exil0867]

Currently, Trena requires an email address for user signup.

Longer term, it may be desirable to support accounts without email addresses
(e.g. username-only signups or non-OAuth flows), which would allow more flexible
identity models and reduce coupling between user identity and email.

However, this raises clear concerns around:

• Account recovery
• Password reset mechanisms
• Long-term account access guarantees

Questions to resolve:

• Is email a permanent requirement for Trena accounts?
• If email becomes optional, what recovery guarantees are acceptable?
• Should email be treated as an optional recovery channel rather than identity?

No changes are planned until these trade-offs are explicitly decided.