Identity: add resend cooldown + verification attempt throttling for email and phone

[antosubash]

Summary

Identity verification endpoints currently have no resend cooldowns or brute-force protection. This was raised on PR #198 by Copilot's automated review and the security argument generalizes to the email flow as well, so we're filing it as a single follow-up rather than landing a phone-only mitigation.

Affected endpoints

Email (already merged):

• POST /Identity/Account/Manage/Email — re-sends change-email confirmation link; no cooldown.
• POST /Identity/Account/ResendEmailConfirmation — anonymous resend; no cooldown.

Phone (added in #198 / #174):

• POST /Identity/Account/Manage/SendPhoneVerificationCode — generates token, dispatches via ISmsSender. No cooldown. Risk: signed-in user can pump SMS to arbitrary numbers, driving provider costs.
• POST /Identity/Account/Manage/ConfirmPhoneNumber — accepts 6-digit code. No failed-attempt tracking. Risk: signed-in attacker (or stolen session) can brute-force ChangePhoneNumberAsync until a code lands.

What we want

1. Resend cooldown — per-user, per-channel (email or phone), e.g. 60 seconds between sends. Probably backed by IDistributedCache so it works across processes.
2. Attempt cap on code submission — for both email-confirmation links (less critical, signed token) and phone 6-digit codes (critical, narrow search space). Lock further attempts for N minutes after K failures.
3. Auditing — emit an AuditLogs event on resend and on failed verification so abuse is visible.

Notes

• Token TTL is already short (default 1h for DataProtectionTokenProvider), but that does nothing against a fast brute-forcer of 6 digits.
• We have SimpleModule.RateLimiting available — route-level limits would catch the most blatant cases for free, but won't stop a single authenticated session pacing itself under the threshold. The real fix is per-user/per-channel counters.
• Out of scope for Identity: phone number confirmation flow #174 (phone parity); shipping that PR first and opening this so neither flow gets forgotten.