Add version history with rollback and per-note created/synced info

Server (schema v2, auto-migrates): every accepted overwrite snapshots the
superseded revision into note_history (capped at 50 per note); new
GET /api/v1/notes/{id}/history endpoint; compact purges orphaned history.

Client: notes get a synced_at stamp on every confirmed server exchange;
an info footer under the editor and a Ctrl/Cmd+I panel show created/
modified/last-synced plus the revision list. Restoring a revision applies
it as a normal edit through the sync path, so rollback is non-destructive.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/api/api_test.go

@@ -229,6 +229,45 @@ func TestTwoClientsConverge(t *testing.T) {
 	}
 }
 
+func TestHistoryEndpoint(t *testing.T) {
+	srv, tok := newTestServer(t)
+	a := newClient(t, srv.URL, tok)
+
+	a.edit("n1", "v1")
+	a.sync()
+	a.edit("n1", "v2")
+	a.notes["n1"].ModifiedAt = 2
+	a.sync()
+
+	var body struct {
+		Revisions []store.Revision `json:"revisions"`
+	}
+	if code := a.req("GET", "/api/v1/notes/n1/history", nil, &body); code != 200 {
+		t.Fatalf("history HTTP %d", code)
+	}
+	if len(body.Revisions) != 1 || body.Revisions[0].Content != "v1" {
+		t.Fatalf("history: %+v", body.Revisions)
+	}
+
+	// Unknown note → empty list, not an error.
+	if code := a.req("GET", "/api/v1/notes/nope/history", nil, &body); code != 200 {
+		t.Fatalf("unknown note HTTP %d", code)
+	}
+	if len(body.Revisions) != 0 {
+		t.Fatalf("want empty history, got %+v", body.Revisions)
+	}
+
+	// Auth required.
+	res, err := http.Get(srv.URL + "/api/v1/notes/n1/history")
+	if err != nil {
+		t.Fatal(err)
+	}
+	res.Body.Close()
+	if res.StatusCode != http.StatusUnauthorized {
+		t.Fatalf("want 401, got %d", res.StatusCode)
+	}
+}
+
 func TestDeleteEditConflictAcrossClients(t *testing.T) {
 	srv, tok := newTestServer(t)
 	a := newClient(t, srv.URL, tok)