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/services/notifier.py

@@ -86,13 +86,8 @@ async def _send_telegram_broadcast(target: NotificationTarget, message: str, rec
     if not bot_token:
         return {"success": False, "error": "Missing bot_token"}
 
-    # Fall back to legacy chat_id if no receivers
     if not receivers:
-        chat_id = target.config.get("chat_id")
-        if chat_id:
-            receivers = [{"chat_id": str(chat_id)}]
-        else:
-            return {"success": False, "error": "No receivers configured"}
+        return {"success": False, "error": "No receivers configured"}
 
     results: list[dict] = []
     async with aiohttp.ClientSession() as session:
@@ -121,14 +116,8 @@ async def _send_telegram_broadcast(target: NotificationTarget, message: str, rec
 async def _send_webhook_broadcast(target: NotificationTarget, message: str, receivers: list[dict]) -> dict:
     from notify_bridge_core.notifications.webhook.client import WebhookClient
 
-    # Fall back to legacy url if no receivers
     if not receivers:
-        url = target.config.get("url")
-        headers = target.config.get("headers", {})
-        if url:
-            receivers = [{"url": url, "headers": headers}]
-        else:
-            return {"success": False, "error": "No receivers configured"}
+        return {"success": False, "error": "No receivers configured"}
 
     results: list[dict] = []
     async with aiohttp.ClientSession() as session:
@@ -206,11 +195,7 @@ async def _send_email_broadcast(target: NotificationTarget, message: str, receiv
 async def _send_webhook_like_broadcast(target: NotificationTarget, message: str, receivers: list[dict]) -> dict:
     """Broadcast for Discord and Slack — both use webhook URLs as receivers."""
     if not receivers:
-        webhook_url = target.config.get("webhook_url")
-        if webhook_url:
-            receivers = [{"webhook_url": webhook_url}]
-        else:
-            return {"success": False, "error": "No receivers configured"}
+        return {"success": False, "error": "No receivers configured"}
 
     results: list[dict] = []
     async with aiohttp.ClientSession() as session:
@@ -238,11 +223,7 @@ async def _send_ntfy_broadcast(target: NotificationTarget, message: str, receive
     auth_token = target.config.get("auth_token")
 
     if not receivers:
-        topic = target.config.get("topic")
-        if topic:
-            receivers = [{"topic": topic}]
-        else:
-            return {"success": False, "error": "No receivers configured"}
+        return {"success": False, "error": "No receivers configured"}
 
     from notify_bridge_core.notifications.ntfy.client import NtfyClient
     results: list[dict] = []
@@ -307,6 +288,26 @@ def _aggregate(results: list[dict]) -> dict:
 # --- Public API used by routes ---
 
 
+async def send_to_receiver(target: NotificationTarget, receiver_config: dict, message: str) -> dict:
+    """Send a message to a single receiver of a target."""
+    try:
+        send_fn = {
+            "telegram": _send_telegram_broadcast,
+            "webhook": _send_webhook_broadcast,
+            "email": _send_email_broadcast,
+            "discord": _send_webhook_like_broadcast,
+            "slack": _send_webhook_like_broadcast,
+            "ntfy": _send_ntfy_broadcast,
+            "matrix": _send_matrix_broadcast,
+        }.get(target.type)
+        if send_fn:
+            return await send_fn(target, message, [receiver_config])
+        return {"success": False, "error": f"Unknown target type: {target.type}"}
+    except Exception as e:
+        _LOGGER.error("Send to receiver failed: %s", e)
+        return {"success": False, "error": str(e)}
+
+
 async def send_test_notification(target: NotificationTarget, locale: str = "en") -> dict:
     """Send a simple test message."""
     message = _get_test_message(locale, target.type)