feat(chat): conversations persistantes + mémoire multi-tour (v0.36.0)

Store SQLite conversations/messages (propriété par actor, fenêtre 20),
historique rejoué au LLM (runAgentTurn history), ChatService persiste et
renvoie conversationId. API GET/DELETE /conversations + chat avec
conversationId. UI Chat: sidebar conversations (drawer mobile), nouvelle,
reprise, suppression. docs/conversations.md. 83 tests verts, build web vert.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016w5jRe87MGdd6AMvXQcHNi

orchestrator/test/api.test.ts

@@ -19,7 +19,7 @@ const auth: AuthConfig = {
 
 // ChatService stub : pas de vrai LLM.
 const chatStub = {
-  handle: async (_actor: string, text: string) => `echo:${text}`,
+  handle: async (_actor: string, text: string) => ({ reply: `echo:${text}`, conversationId: null }),
 } as unknown as ChatService;
 
 let app: FastifyInstance;
@@ -29,7 +29,7 @@ beforeEach(async () => {
   repo = new AssetRepository(":memory:");
   const review = new ReviewService(repo, log);
   app = Fastify();
-  await registerApi(app, { auth, chat: chatStub, review, state: () => ({ phase: "2-write-review", tools: 3 }) });
+  await registerApi(app, { auth, chat: chatStub, review, conversations: null, state: () => ({ phase: "2-write-review", tools: 3 }) });
   await app.ready();
 });
 afterEach(async () => {