fix(alerts,data): startup grace + sustained offline + atomic JSON writes

- 5 min sustained offline window для VPS/PC/Synology/HA (раньше было только у
  Keenetic) + новый STARTUP_GRACE_SEC=300 — первые 5 минут после старта сервиса
  алёрты offline не отправляются, но состояние трекается. Это убирает массовый
  fantom-alert при перезапуске vps-monitoring.
- Атомарная запись JSON-файлов через tempfile+os.replace и явный encoding=utf-8
  везде (data/keenetic.json, pc_agents.json, synology.json, homeassistant.json,
  servers.json, settings.json, metrics.json). Краш в момент записи больше не
  оставляет битый файл с русскими названиями.
- Чистка: убран неиспользуемый passlib + dead bcrypt-импорт в auth.py
  (verify_password всё равно был обычным сравнением); SECRET_KEY можно
  переопределить через VPS_MONITORING_SECRET; убран мертвый import json там,
  где он больше не нужен.
- Bump app.js cache bust до 20260524c.

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

server/api/ha.py

@@ -1,13 +1,12 @@
 """Home Assistant monitoring API endpoints."""
 
-import json
 from datetime import datetime
 from typing import Dict
 
 from fastapi import APIRouter, Request, Depends
 
 from server.auth import require_auth
-from server.config import DATA_DIR
+from server.config import DATA_DIR, load_json, save_json
 
 router = APIRouter(prefix="/api/ha", tags=["homeassistant"])
 
@@ -18,15 +17,11 @@ HA_FILE = DATA_DIR / "homeassistant.json"
 
 
 def _load_ha():
-    if HA_FILE.exists():
-        with open(HA_FILE) as f:
-            return json.load(f)
-    return []
+    return load_json(HA_FILE, [])
 
 
 def _save_ha(data):
-    with open(HA_FILE, "w") as f:
-        json.dump(data, f, indent=2, ensure_ascii=False)
+    save_json(HA_FILE, data)
 
 
 @router.get("/list")

server/api/keenetic.py

@@ -1,7 +1,6 @@
 """Keenetic router monitoring API endpoints."""
 
 import asyncio
-import json
 import logging
 from datetime import datetime
 from typing import Dict, List
@@ -10,7 +9,7 @@ from urllib.parse import urlparse
 from fastapi import APIRouter, Request, Depends
 
 from server.auth import require_auth
-from server.config import DATA_DIR, load_settings
+from server.config import DATA_DIR, load_settings, load_json, save_json
 from server.services.keenetic_client import (
     KeeneticClient,
     normalize_web_url,
@@ -31,15 +30,11 @@ REFRESH_ALL_GAP_SEC = 2
 
 
 def _load_keenetic():
-    if KEENETIC_FILE.exists():
-        with open(KEENETIC_FILE) as f:
-            return json.load(f)
-    return []
+    return load_json(KEENETIC_FILE, [])
 
 
 def _save_keenetic(data):
-    with open(KEENETIC_FILE, "w") as f:
-        json.dump(data, f, indent=2, ensure_ascii=False)
+    save_json(KEENETIC_FILE, data)
 
 
 def _host_from_url(url: str) -> str:

server/api/pc.py

@@ -1,11 +1,10 @@
-import json
 from datetime import datetime
 from typing import Dict
 
 from fastapi import APIRouter, Request, Depends
 
 from server.auth import require_auth
-from server.config import DATA_DIR
+from server.config import DATA_DIR, load_json, save_json
 
 router = APIRouter(prefix="/api/pc", tags=["pc"])
 
@@ -16,15 +15,11 @@ PC_FILE = DATA_DIR / "pc_agents.json"
 
 
 def _load_pc_data():
-    if PC_FILE.exists():
-        with open(PC_FILE) as f:
-            return json.load(f)
-    return {}
+    return load_json(PC_FILE, {})
 
 
 def _save_pc_data(data):
-    with open(PC_FILE, "w") as f:
-        json.dump(data, f, indent=2, ensure_ascii=False)
+    save_json(PC_FILE, data)
 
 
 @router.post("/heartbeat")

server/api/synology.py

@@ -1,16 +1,15 @@
 """Synology NAS monitoring API endpoints."""
 
-import json
 import os
 from datetime import datetime
 from pathlib import Path
-from typing import Dict, List
+from typing import Dict
 
 from fastapi import APIRouter, Request, Depends
 from fastapi.responses import PlainTextResponse
 
 from server.auth import require_auth
-from server.config import DATA_DIR, BASE_DIR
+from server.config import DATA_DIR, BASE_DIR, load_json, save_json
 
 router = APIRouter(prefix="/api/synology", tags=["synology"])
 
@@ -25,15 +24,11 @@ SYNOLOGY_FILE = DATA_DIR / "synology.json"
 
 
 def _load_synology():
-    if SYNOLOGY_FILE.exists():
-        with open(SYNOLOGY_FILE) as f:
-            return json.load(f)
-    return []
+    return load_json(SYNOLOGY_FILE, [])
 
 
 def _save_synology(data):
-    with open(SYNOLOGY_FILE, "w") as f:
-        json.dump(data, f, indent=2, ensure_ascii=False)
+    save_json(SYNOLOGY_FILE, data)
 
 
 @router.get("/list")