feat(telegram): per-chat command localization + unified locale resolver

Two related Telegram changes:

1. Per-chat command localization. setMyCommands now accepts a scope
   (BotCommandScopeChat) and deleteMyCommands clears scoped bindings.
   Command registration runs three tiers: default → per-language
   (Telegram client language) → per-chat (UI override). Saving a
   chat's language_override or commands_enabled toggle pushes the
   binding to Telegram inline rather than waiting on the 30s
   debounced bot-wide sync.

2. Unified Telegram locale resolution. Three test paths (bot test_chat,
   target receiver test, target-level fan-out) used to disagree on
   locale priority — the target receiver test in particular only
   consulted receiver.locale and ignored the chat's language_override.
   Introduced pick_telegram_locale (pure) and
   resolve_telegram_chat_locale (async DB lookup) in services/notifier
   so all three paths share one priority order:

       receiver.locale → chat.language_override → chat.language_code → fallback

   Fan-out keeps batch-loading TelegramChat rows for efficiency, just
   runs them through the same priority function now.

packages/server/src/notify_bridge_server/services/notifier.py

@@ -43,6 +43,62 @@ def _get_test_message(locale: str, target_type: str) -> str:
     return msgs.get(target_type, msgs.get("webhook", "Test"))
 
 
+def pick_telegram_locale(
+    *,
+    receiver_locale: str = "",
+    chat_override: str = "",
+    chat_language_code: str = "",
+    fallback: str = "en",
+) -> str:
+    """Pick the effective 2-letter locale for a Telegram chat.
+
+    Priority (highest first):
+        1. ``receiver_locale``     — explicit per-receiver override on a target.
+        2. ``chat_override``       — explicit ``TelegramChat.language_override``
+                                     set in the bot/chat manager UI.
+        3. ``chat_language_code``  — Telegram-provided ``language_code``.
+        4. ``fallback``            — caller-supplied default (e.g. query param).
+
+    All inputs are coerced to lowercase 2-letter codes.
+    """
+    for candidate in (receiver_locale, chat_override, chat_language_code, fallback):
+        if candidate:
+            return candidate[:2].lower()
+    return "en"
+
+
+async def resolve_telegram_chat_locale(
+    session: AsyncSession,
+    *,
+    bot_id: int | None,
+    chat_id: str | int | None,
+    receiver: TargetReceiver | None = None,
+    fallback: str = "en",
+) -> str:
+    """Look up a Telegram chat and resolve its effective locale.
+
+    Single source of truth for "what language should I send to this chat in?".
+    Used by every Telegram test/preview path (bot test_chat, target test
+    receiver, per-receiver fan-out) so they stay in lockstep.
+    """
+    from ..database.models import TelegramChat
+
+    chat_row = None
+    if bot_id and chat_id:
+        chat_row = (await session.exec(
+            select(TelegramChat).where(
+                TelegramChat.bot_id == bot_id,
+                TelegramChat.chat_id == str(chat_id),
+            )
+        )).first()
+    return pick_telegram_locale(
+        receiver_locale=getattr(receiver, "locale", "") if receiver else "",
+        chat_override=getattr(chat_row, "language_override", "") if chat_row else "",
+        chat_language_code=getattr(chat_row, "language_code", "") if chat_row else "",
+        fallback=fallback,
+    )
+
+
 async def _load_receivers(target_id: int) -> list[dict]:
     """Load enabled receivers for a target from DB."""
     engine = get_engine()
@@ -343,9 +399,12 @@ async def _send_telegram_test_per_receiver(
         if not recv_rows:
             return {"success": False, "error": "No receivers configured"}
 
-        # Resolve per-receiver locale
+        # Batch-load TelegramChat rows so per-receiver locale picks don't
+        # round-trip the DB N times. Priority resolution then runs through the
+        # shared pick_telegram_locale() helper so single-shot test endpoints
+        # and this fan-out agree on the same rules.
         chat_ids = [str(r.config.get("chat_id", "")) for r in recv_rows if r.config.get("chat_id")]
-        chat_locale_map: dict[str, str] = {}
+        chat_row_map: dict[str, TelegramChat] = {}
         if bot_id and chat_ids:
             chat_rows = (await session.exec(
                 select(TelegramChat).where(
@@ -353,13 +412,7 @@ async def _send_telegram_test_per_receiver(
                     TelegramChat.chat_id.in_(chat_ids),
                 )
             )).all()
-            for chat in chat_rows:
-                override = (
-                    getattr(chat, "language_override", "") or
-                    getattr(chat, "language_code", "") or ""
-                )
-                if override:
-                    chat_locale_map[chat.chat_id] = override[:2].lower()
+            chat_row_map = {chat.chat_id: chat for chat in chat_rows}
 
     http = await get_http_session()
     client = TelegramClient(http, bot_token)
@@ -374,9 +427,14 @@ async def _send_telegram_test_per_receiver(
         chat_id = str(r.config.get("chat_id", ""))
         if not chat_id:
             return None
-        explicit = getattr(r, "locale", "") or ""
-        locale = explicit or chat_locale_map.get(chat_id) or default_locale
-        message = _get_test_message(locale[:2].lower(), "telegram")
+        chat_row = chat_row_map.get(chat_id)
+        locale = pick_telegram_locale(
+            receiver_locale=getattr(r, "locale", "") or "",
+            chat_override=getattr(chat_row, "language_override", "") if chat_row else "",
+            chat_language_code=getattr(chat_row, "language_code", "") if chat_row else "",
+            fallback=default_locale,
+        )
+        message = _get_test_message(locale, "telegram")
         async with sem:
             return await client.send_message(
                 chat_id=chat_id,