feat: comprehensive code review fixes + receivers-only architecture

Security:
- Refuse startup with default secret_key in production (was just logging)
- Settings endpoint now requires admin role
- Password validation on initial setup
- DOM-based HTML sanitizer replaces regex in template previews
- Add *.log to .gitignore

Performance & reliability:
- Token refresh deduplication prevents race condition on concurrent 401s
- Theme media query listener registered once (no leak)
- IconPicker uses $derived instead of function call per render
- Snackbar uses single-batch state update instead of while loop
- Replace 11 inline hover handlers with CSS :hover in layout

Architecture - receivers-only:
- Delivery endpoints (chat_id, email, url, room_id, topic) now stored
  exclusively in TargetReceiver rows, never in target.config
- Migration extracts existing delivery fields to receiver rows
- Notifier and dispatcher remove all config fallbacks
- Frontend targets page shows receivers list per target with
  add/remove/toggle/test per receiver
- Single-receiver test endpoint: POST /targets/{id}/receivers/{id}/test

Code quality:
- Extract AuthLayout.svelte from login/setup (150 lines CSS dedup)
- Split telegram-bots page (754→51 lines + 3 tab components)
- Split notification-trackers page (547→432 lines + 4 components)
- Deduplicate _send_reply into shared handler.send_reply()
- Add locale column to template models, replace name-based detection
- Fix delete_notification_tracker dead protection check
- Fix check_telegram_bot query (filter by type, remove bogus OR)
- Add graceful scheduler shutdown in lifespan
- Consistent /bots?tab=telegram URLs across all nav links

i18n:
- Error page, chat actions, target types, provider types internationalized
- All new receiver UI strings in EN + RU

packages/server/src/notify_bridge_server/api/app_settings.py

@@ -8,7 +8,7 @@ from pydantic import BaseModel
 from sqlmodel import select
 from sqlmodel.ext.asyncio.session import AsyncSession
 
-from ..auth.dependencies import get_current_user
+from ..auth.dependencies import require_admin
 from ..database.engine import get_session
 from ..database.models import AppSetting, TelegramBot, User
 
@@ -51,7 +51,7 @@ class SettingsUpdate(BaseModel):
 
 @router.get("")
 async def get_settings(
-    user: User = Depends(get_current_user),
+    user: User = Depends(require_admin),
     session: AsyncSession = Depends(get_session),
 ):
     """Return all app settings."""
@@ -64,7 +64,7 @@ async def get_settings(
 @router.put("")
 async def update_settings(
     body: SettingsUpdate,
-    user: User = Depends(get_current_user),
+    user: User = Depends(require_admin),
     session: AsyncSession = Depends(get_session),
 ):
     """Update app settings (admin). Re-registers webhooks when base URL changes."""