fix(chat): make cloud the only truth for charts, synastry, and pin/archive

Local fallbacks were creating fake saves and resurrecting deleted rows.
Pin and archive now live on chat_sessions so they follow the account.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/tests/chat-session-write.test.ts

@@ -1,7 +1,7 @@
 import assert from "node:assert/strict";
 import { readFileSync } from "node:fs";
 import test from "node:test";
-import { chatSessionCreateSchema, chatSessionWriteSchema, writeChatSession, type ChatSessionWrite } from "../src/lib/chat-session-write-contract.ts";
+import { chatSessionCreateSchema, chatSessionMetadataPatchSchema, chatSessionWriteSchema, writeChatSession, type ChatSessionWrite } from "../src/lib/chat-session-write-contract.ts";
 
 const sessionId = "11111111-1111-4111-8111-111111111111";
 const values = {
@@ -111,6 +111,15 @@ test("chat session schema keeps structured thinking sections on assistant messag
   assert.equal(parsed.messages[0]?.thinkingSections?.[0]?.heading, "统一参数与原始结构");
 });
 
+test("metadata patch accepts pin and archive fields without a transcript", () => {
+  assert.deepEqual(chatSessionMetadataPatchSchema.parse({ pinned: true }), { pinned: true });
+  assert.deepEqual(
+    chatSessionMetadataPatchSchema.parse({ archived_at: "2026-09-01T00:00:00.000Z" }),
+    { archived_at: "2026-09-01T00:00:00.000Z" },
+  );
+  assert.deepEqual(chatSessionMetadataPatchSchema.parse({ archived_at: null }), { archived_at: null });
+});
+
 test("chat session writes use same-origin API instead of browser-to-Supabase requests", async () => {
   const calls: Array<{ url: string; init?: RequestInit }> = [];
   await writeChatSession(sessionId, metadataPatch, "update", async (url, init) => {