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.mod

@@ -7,6 +7,8 @@ require (
 	github.com/spf13/cobra v1.10.2
 	golang.org/x/crypto v0.49.0
 	golang.org/x/term v0.41.0
+	google.golang.org/grpc v1.79.3
+	google.golang.org/protobuf v1.36.11
 	modernc.org/sqlite v1.47.0
 )
 
@@ -18,7 +20,10 @@ require (
 	github.com/ncruces/go-strftime v1.0.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
 	github.com/spf13/pflag v1.0.9 // indirect
+	golang.org/x/net v0.51.0 // indirect
 	golang.org/x/sys v0.42.0 // indirect
+	golang.org/x/text v0.35.0 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20251202230838-ff82c1b0f217 // indirect
 	modernc.org/libc v1.70.0 // indirect
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect