Signup allows disposable/temporary email domains, enabling multiple fake accounts

### 🚀 The feature

While reviewing the authentication design, I noticed that the platform intentionally supports disposable/temporary email registration as part of its privacy-first goal (to keep real mailboxes clean and allow anonymous usage).

This aligns well with the stated motivation of protecting users from spam. At the same time, it introduces a clear trade-off in terms of user identity strength, making it difficult to enforce per-user limits, quotas, or one-user-per-account policies in certain deployments.

**Actual behavior:**
- The platform intentionally allows registration using disposable/temporary email services, aligned with its privacy-first goal.

**Expected / discussion point:**
- Clarification on whether this design trade-off (privacy vs. stronger user identity) is documented and intentional long-term.
- Whether an optional stricter mode (e.g., verified email only) might be useful for deployments that require per-user limits or stronger identity guarantees.

No screenshots are applicable, as this is a design-level discussion rather than a UI or functional issue.

### Motivation, pitch

The motivation for raising this is not to suggest removing this behavior, but to discuss whether:

- This trade-off should be documented more explicitly in the authentication design, and/or
- An optional stricter registration mode (e.g., verified email only) could be useful for deployments that require stronger user identity guarantees.

This is intended as a design discussion rather than a bug report.