Implement Phase 4: gRPC sync service

- Proto definitions (engpad.v1.EngPadSync) with 6 RPCs
- Generated Go gRPC code
- Auth interceptor: username/password from metadata
- SyncNotebook: upsert with full page/stroke replacement in a tx
- DeleteNotebook, ListNotebooks handlers
- Share link RPCs: CreateShareLink, RevokeShareLink, ListShareLinks
- Share link token management (32-byte random, optional expiry)
- gRPC server with TLS 1.3

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

go.sum

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