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

frontend/src/lib/components/SearchPalette.svelte

@@ -51,7 +51,7 @@
 			mapFn: (e: any) => ({ detail: e.provider_type, icon: e.icon || 'mdiFileDocumentEdit' }) },
 		{ key: 'targets', label: 'nav.targets', icon: 'mdiTarget', href: '/targets',
 			mapFn: (e: any) => ({ detail: e.type, icon: e.icon || 'mdiTarget' }) },
-		{ key: 'telegram_bots', label: 'nav.telegram', icon: 'mdiSendCircle', href: '/bots',
+		{ key: 'telegram_bots', label: 'nav.telegram', icon: 'mdiSendCircle', href: '/bots?tab=telegram',
 			mapFn: (e: any) => ({ detail: `@${e.bot_username || ''}`, icon: e.icon || 'mdiRobot' }) },
 		{ key: 'email_bots', label: 'nav.email', icon: 'mdiEmailOutline', href: '/bots?tab=email',
 			mapFn: (e: any) => ({ detail: e.email || '', icon: e.icon || 'mdiEmailOutline' }) },