feat(ai-chat): persistent history as source of truth — step durability + server export (#183)

The chat lived in inconsistent paradigms (in-memory stream + client export vs.
DB-as-context), which made export flaky and lost the assistant answer if the
process died mid-turn. Make the DB the single source of truth.

A. STEP-GRANULAR DURABILITY (server)
- ai_chat_messages gains a nullable `status` column (migration; NULL = legacy =
  completed). The assistant row is now INSERTED UPFRONT as `status:'streaming'`
  and UPDATEd on every onStepFinish with all finished steps (text + tool calls +
  tool RESULTS), then finalized once to completed/error/aborted on the terminal
  callback. So a process death mid-turn keeps every finished step; a startup
  sweep (OnModuleInit → sweepStreaming) flips any dangling 'streaming' row to
  'aborted'. The write path no longer depends on a live socket.
- Pure exported `flushAssistant(steps, inProgressText, status, extra?)` builds
  the persist payload (metadata.parts byte-identical to the old builder), so a
  future background worker can call the same path. AiChatMessageRepo gains
  `update`, `sweepStreaming`, and `findAllByChat`.
- consumeStream drain, external-MCP client close-once, SSE heartbeat preserved.

B. SERVER-SIDE EXPORT
- New pure `chat-markdown.util.ts` renders Markdown from DB rows ONLY (server
  port of the client builder). Because A persists the in-progress row, the
  export now includes an interrupted turn up to its last finished step (flagged
  "still generating"). `POST /ai-chat/export` (owner-gated via assertOwnedChat,
  workspace-scoped) returns it; `lang` accepts a full client locale tag
  ('en-US'/'ru-RU') and is normalized server-side (normalizeLang) — a strict
  @IsIn(['en','ru']) DTO rejected the real client's i18n.language with a 400,
  caught in real-browser testing.
- Client: handleCopy calls the endpoint; `canExport = !!activeChatId`. The whole
  liveThreadRef/liveStateRef/onLiveContentChange/hasLiveContent hybrid (and the
  client chat-markdown util + test) is removed — the server is now authoritative.

Tests: flushAssistant unit (status shapes + parts parity), chat-markdown.util
unit (incl. legacy NULL-status + interrupted note + ru + normalizeLang locale
tags), controller export wiring + owner-gate, integration update/sweepStreaming.
Verified: server build + 318 ai-chat unit + 3 integration; client tsc + 157
ai-chat unit; and END-TO-END in a real browser — a chat turn persists mid-stream
and the Copy button exports the DB-sourced markdown (showing the in-progress
row), HTTP 200 after the locale fix.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/test/integration/db.ts

@@ -104,7 +104,8 @@ export async function createWorkspace(
       name: overrides.name ?? `ws-${suffix}`,
       // hostname is uniquely constrained; keep it unique per workspace.
       hostname: `host-${suffix}`,
-      settings: overrides.settings === undefined ? null : (overrides.settings as any),
+      settings:
+        overrides.settings === undefined ? null : (overrides.settings as any),
     })
     .returning(['id', 'settings'])
     .executeTakeFirstOrThrow();
@@ -226,3 +227,33 @@ export async function createChat(
     .executeTakeFirstOrThrow();
   return { id: row.id as string };
 }
+
+export async function createMessage(
+  db: Kysely<any>,
+  args: {
+    workspaceId: string;
+    chatId: string;
+    userId?: string | null;
+    role?: string;
+    content?: string | null;
+    status?: string | null;
+    metadata?: unknown;
+  },
+): Promise<{ id: string }> {
+  const id = randomUUID();
+  const row = await db
+    .insertInto('aiChatMessages')
+    .values({
+      id,
+      workspaceId: args.workspaceId,
+      chatId: args.chatId,
+      userId: args.userId ?? null,
+      role: args.role ?? 'assistant',
+      content: args.content ?? null,
+      status: args.status ?? null,
+      metadata: (args.metadata ?? null) as any,
+    })
+    .returning(['id'])
+    .executeTakeFirstOrThrow();
+  return { id: row.id as string };
+}