fix(mail): SMTP send retry (2x2s) + 502 on verification-code send failure (rant 2026-08-21T23:52:17)

[argszero]

Summary

Fault tolerance for the verification-code email path per host rant 2026-08-21T23:52:17 (project=aitokenpool). Gmail (and similar providers) intermittently and silently drop connections from datacenter IPs (Alibaba/Tencent Cloud etc.) — TCP/TLS succeed but the SMTP banner never arrives, lettre times out after 15s, HTTP 500. This is external-service unreliability, so the fix is app-level:

• SMTP send retry (src/mail.rs): new send_with_retry helper — 1 initial attempt + up to 2 retries with a fixed 2s delay, fresh transport per attempt (new TCP+TLS connection, matching the observed transient-drop pattern). Errors only after all 3 attempts fail.
• Semi-registration fallback (src/routes/mod.rs send_code): on final SMTP failure, the verification record is cleared (releases the 60s resend rate limit, so the user can resend immediately instead of being stuck) and the API returns 502 + 「验证码发送失败，请重试」 (was 500) — register/resend/forgot-password all benefit.

Related Issue

Host rant 2026-08-21T23:52:17.109049+08:00 (project=aitokenpool), verbatim:

SMTP 发送验证码间歇性超时 + 半注册账号卡死问题（线上 dev 实测）

建议修复

1. SMTP 发送加重试（核心）：发送失败（Connection/TimedOut 类错误）后延迟 2-3s 重试，最多 2 次；重试仍失败才报错返回；重试间隔用固定 2s 即可
2. 半注册账号兜底（连带问题）：注册时验证码发送失败 → 建议：注册接口发送失败时返回明确错误（如 502 + 「验证码发送失败，请重试」），并允许该邮箱重新注册/重发；检查 resend-code 对 verified=0 账号是否已有 60s 限频（若有，失败后的重发不要被限频卡住）
3. 测试：模拟 SMTP 连接失败 → 断言重试后成功；mock 恒失败 → 断言最终报错且不无限重试

Changes

• SMTP retry: up to 2 retries × fixed 2s delay, fresh transport per attempt
• 502 + clear error on final send failure (was 500)
• Verification record cleared on failure → resend not blocked by the 60s rate limit
• Tests: retry succeeds after transient failures / no infinite retry / last-error propagated (3 unit tests) + register-smtp-failure → 502 integration test (resend right after is not rate-limited)

Tests

• cargo test — 136/136 passed (132 + 4 new)
• cargo fmt --check — clean
• cargo clippy --all-targets -- -D warnings — clean

Checklist

• Branch name follows convention (fix/smtp-retry)
• Commit message uses Conventional Commits
• Single responsibility, minimal change