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>

proto/engpad/v1/sync.proto

@@ -0,0 +1,94 @@
+syntax = "proto3";
+package engpad.v1;
+
+option go_package = "git.wntrmute.dev/kyle/eng-pad-server/gen/engpad/v1;engpadv1";
+
+import "google/protobuf/timestamp.proto";
+
+service EngPadSync {
+    rpc SyncNotebook(SyncNotebookRequest) returns (SyncNotebookResponse);
+    rpc DeleteNotebook(DeleteNotebookRequest) returns (DeleteNotebookResponse);
+    rpc ListNotebooks(ListNotebooksRequest) returns (ListNotebooksResponse);
+    rpc CreateShareLink(CreateShareLinkRequest) returns (CreateShareLinkResponse);
+    rpc RevokeShareLink(RevokeShareLinkRequest) returns (RevokeShareLinkResponse);
+    rpc ListShareLinks(ListShareLinksRequest) returns (ListShareLinksResponse);
+}
+
+message SyncNotebookRequest {
+    int64 notebook_id = 1;
+    string title = 2;
+    string page_size = 3;
+    repeated PageData pages = 4;
+}
+
+message PageData {
+    int64 page_id = 1;
+    int32 page_number = 2;
+    repeated StrokeData strokes = 3;
+}
+
+message StrokeData {
+    float pen_size = 1;
+    int32 color = 2;
+    string style = 3;
+    bytes point_data = 4;
+    int32 stroke_order = 5;
+}
+
+message SyncNotebookResponse {
+    int64 server_notebook_id = 1;
+    google.protobuf.Timestamp synced_at = 2;
+}
+
+message DeleteNotebookRequest {
+    int64 notebook_id = 1;
+}
+
+message DeleteNotebookResponse {}
+
+message ListNotebooksRequest {}
+
+message ListNotebooksResponse {
+    repeated NotebookSummary notebooks = 1;
+}
+
+message NotebookSummary {
+    int64 server_id = 1;
+    int64 remote_id = 2;
+    string title = 3;
+    string page_size = 4;
+    int32 page_count = 5;
+    google.protobuf.Timestamp synced_at = 6;
+}
+
+message CreateShareLinkRequest {
+    int64 notebook_id = 1;
+    int64 expires_in_seconds = 2;
+}
+
+message CreateShareLinkResponse {
+    string token = 1;
+    string url = 2;
+    google.protobuf.Timestamp expires_at = 3;
+}
+
+message RevokeShareLinkRequest {
+    string token = 1;
+}
+
+message RevokeShareLinkResponse {}
+
+message ListShareLinksRequest {
+    int64 notebook_id = 1;
+}
+
+message ListShareLinksResponse {
+    repeated ShareLinkInfo links = 1;
+}
+
+message ShareLinkInfo {
+    string token = 1;
+    string url = 2;
+    google.protobuf.Timestamp created_at = 3;
+    google.protobuf.Timestamp expires_at = 4;
+}