Add multi-user auth, background generation, and chat UX improvements

Phase 5: Multi-user authentication with session cookies, bcrypt passwords,
first-user-is-admin pattern, per-user data isolation, backup/restore,
Docker Swarm production stack with secrets and network isolation.

Phase 5.1: Chat UX improvements:
- Background generation architecture (GenerationBuffer + asyncio task)
  with SSE fan-out, reconnect support, and periodic DB flushes
- LLM-generated conversation titles (first exchange + every 10th message)
- Stop generation button with cancel_event and partial content preservation
- Relative timestamps in sidebar (5m ago, 3h ago, then dates)
- Empty chat auto-cleanup on navigation away
- Save-as-note uses LLM for title generation, tags notes with "chat"
- Summarize-as-note also tags with "chat"

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/types/auth.ts

@@ -0,0 +1,11 @@
+export interface User {
+  id: number;
+  username: string;
+  email: string | null;
+  role: string;
+  created_at: string;
+}
+
+export interface AuthStatus {
+  has_users: boolean;
+}

frontend/src/types/chat.ts

@@ -3,10 +3,16 @@ export interface Message {
   conversation_id: number;
   role: "system" | "user" | "assistant";
   content: string;
+  status?: "complete" | "generating" | "error";
   context_note_id: number | null;
   created_at: string;
 }
 
+export interface SendMessageResponse {
+  assistant_message_id: number;
+  status: string;
+}
+
 export interface Conversation {
   id: number;
   title: string;