feat(auth): SmartAuth backend core — OAuth, MFA, passkeys, device trust, login events
Phase 0A: OneAuth schema extension — UserDoc evolution + auth_providers container
Phase 0B: Progressive lockout + IP rate limiting on login
Phase 1A-1B: Google/Microsoft/Apple OAuth + account linking
Phase 1D: Enhanced /auth/me — products, providers, MFA status
Phase 2A-2C: TOTP MFA + challenge flow + admin policies
Phase 3A-3B: WebAuthn passkeys + device trust
Phase 4A: Login events + rule-based risk scoring
New sub-modules: oauth/, mfa/, passkeys/, devices/, login-events/
New containers: auth_providers, auth_mfa, auth_mfa_policies, auth_passkeys, auth_devices, auth_login_events
Tests: 37 new (946 total, all passing), typecheck clean
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