Add phone auth and OTP support

[kacy]

Summary

• Add signup_phone_password, sign_in_otp, verify_otp, and resend_otp methods to the auth module
• Add corresponding request structs (PhoneCredentials, OtpRequest, VerifyOtpRequest, ResendOtpRequest)
• Add tests for all 4 new methods following the existing graceful-skip pattern
• Mark 4 README checklist items as complete: phone signup, verify OTP, OTP authentication, resend OTP

Test plan

• cargo build --lib compiles cleanly
• cargo test — all 23 tests pass
• Existing auth and database tests unaffected