Register Telegram as a platform: Platform/ContentType enum cases, a
platforms.telegram config block (shared bot token via env), TelegramPublisher
(sendMessage / sendPhoto|Video|Document / sendMediaGroup over the Bot API, HTML
parse mode, 4096 limit with long text split off a 1024 caption), wired into the
publisher dispatch. Add a Telegram ContentSanitizer branch (Telegram-allowed
HTML + ampersand escaping), MediaOptimizer/profile-url/factory support, and the
TelegramPublishException. Tests cover text, single media, album, long-text split,
overflow, API errors, private-channel URLs, and sanitization.
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