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>

src/fabledassistant/routes/admin.py

@@ -0,0 +1,47 @@
+import json
+
+from quart import Blueprint, Response, jsonify, request
+
+from fabledassistant.auth import admin_required, login_required, get_current_user_id
+from fabledassistant.services.backup import (
+    export_full_backup,
+    export_user_backup,
+    restore_full_backup,
+)
+
+admin_bp = Blueprint("admin", __name__, url_prefix="/api/admin")
+
+
+@admin_bp.route("/backup", methods=["GET"])
+@login_required
+async def backup():
+    uid = get_current_user_id()
+    scope = request.args.get("scope", "user")
+
+    if scope == "full":
+        # Full backup requires admin
+        from quart import g
+        if g.user.role != "admin":
+            return jsonify({"error": "Admin access required for full backup"}), 403
+        data = await export_full_backup()
+    else:
+        data = await export_user_backup(uid)
+
+    return Response(
+        json.dumps(data, indent=2, default=str),
+        content_type="application/json",
+        headers={
+            "Content-Disposition": f'attachment; filename="fabled-backup-{scope}.json"',
+        },
+    )
+
+
+@admin_bp.route("/restore", methods=["POST"])
+@admin_required
+async def restore():
+    data = await request.get_json()
+    if not data:
+        return jsonify({"error": "No backup data provided"}), 400
+
+    stats = await restore_full_backup(data)
+    return jsonify({"status": "ok", "stats": stats})