feat: Receiver OOP hierarchy with per-receiver locale resolution

- Introduce Receiver base class + typed subclasses (TelegramReceiver,
  WebhookReceiver, EmailReceiver, etc.) in core/notifications/receiver.py
- Dispatcher uses typed Receiver objects instead of raw dicts, with
  per-receiver locale-aware template rendering
- load_link_data resolves locale from TelegramChat.language_override at
  load time: TargetReceiver.locale || chat.language_override || chat.language_code
- Add language_override field to TelegramChat (separate from auto-detected
  language_code), with per-chat commands toggle and command dispatch using
  override language
- Add locale field to TargetReceiver for explicit per-receiver overrides

frontend/src/routes/bots/TelegramBotTab.svelte

@@ -129,10 +129,10 @@
 		try {
 			await api(`/telegram-bots/${botId}/chats/${chat.id}`, {
 				method: 'PUT',
-				body: JSON.stringify({ language_code: lang }),
+				body: JSON.stringify({ language_override: lang }),
 			});
 			chats[botId] = (chats[botId] || []).map(c =>
-				c.id === chat.id ? { ...c, language_code: lang } : c
+				c.id === chat.id ? { ...c, language_override: lang } : c
 			);
 			snackSuccess(t('telegramBot.languageUpdated'));
 		} catch (err: any) { snackError(err.message); }
@@ -362,13 +362,14 @@
 						{:else if (chats[bot.id] || []).length === 0}
 							<p class="text-xs text-[var(--color-muted-foreground)]">{t('telegramBot.noChats')}</p>
 						{:else}
-							{@const gridStyle = "display:grid; grid-template-columns:1fr 80px 100px 50px 130px 60px; align-items:center; gap:0.5rem;"}
+							{@const gridStyle = "display:grid; grid-template-columns:1fr 80px 40px 100px 50px 130px 60px; align-items:center; gap:0.5rem;"}
 							<!-- Header -->
 							<div style="{gridStyle} padding:0.25rem 0.5rem; border-bottom:1px solid var(--color-border);"
 								class="text-[0.65rem] font-semibold uppercase tracking-wide text-[var(--color-muted-foreground)]">
 								<span>{t('telegramBot.chatName')}</span>
 								<span style="text-align:center">{t('telegramBot.chatType')}</span>
 								<span style="text-align:center">{t('telegramBot.chatLang')}</span>
+								<span style="text-align:center">{t('telegramBot.langOverride')}</span>
 								<span style="text-align:center">{t('telegramBot.cmds')}</span>
 								<span style="text-align:center">{t('telegramBot.chatId')}</span>
 								<span></span>
@@ -382,10 +383,11 @@
 									role="button" tabindex="0">
 									<span class="font-medium truncate">{chat.title || chat.username || 'Unknown'}</span>
 									<span style="text-align:center" class="text-xs px-1.5 py-0.5 rounded bg-[var(--color-muted)] text-[var(--color-muted-foreground)]">{chatTypeLabel(chat.type)}</span>
+									<span style="text-align:center" class="text-xs text-[var(--color-muted-foreground)]">{(chat.language_code || '—').toUpperCase()}</span>
 									<div style="justify-self:center" onclick={(e: MouseEvent) => e.stopPropagation()}>
 										<EntitySelect
 											items={LANG_ITEMS}
-											value={chat.language_code || ''}
+											value={chat.language_override || ''}
 											size="sm"
 											onselect={(val) => updateChatLanguage(bot.id, chat, String(val ?? ''))}
 										/>