Implement Phase 9: FIDO2/U2F WebAuthn support

- WebAuthnUser implementing webauthn.User interface
- NewWebAuthn factory with configurable RP settings
- Credential storage: store, load, list, delete, update sign count
- User lookup by credential ID for login flow
- go-webauthn/webauthn library integrated

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

go.sum

@@ -1,18 +1,34 @@
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@@ -27,6 +43,8 @@ github.com/ncruces/go-strftime v1.0.0 h1:HMFp8mLCTPp341M/ZnA4qaf7ZlsbTc+miZjCLOF
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@@ -34,6 +52,10 @@ github.com/spf13/cobra v1.10.2 h1:DMTTonx5m65Ic0GOoRY2c16WCbHxOOw6xxezuLaBpcU=
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@@ -46,6 +68,8 @@ go.opentelemetry.io/otel/sdk/metric v1.39.0 h1:cXMVVFVgsIf2YL6QkRF4Urbr/aMInf+2W
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@@ -73,6 +97,8 @@ google.golang.org/grpc v1.79.3/go.mod h1:KmT0Kjez+0dde/v2j9vzwoAScgEPx/Bw1CYChhH
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