fix: deduplicate incoming messages to prevent multiple auto-replies

[octo-patch]

Fixes #75

Problem

The same seller message was being read multiple times (with different timestamps in some cases), causing the bot to generate and send multiple replies for a single incoming message. This resulted in buyers receiving duplicate or repeated responses.

Solution

Add message deduplication using the messageId extracted from the reminderUrl field in each message:

• In __init__, added self.processed_message_ids (a set) and self.max_processed_ids (configurable via MAX_PROCESSED_IDS env var, default 1000) to track recently-processed message IDs.
• In handle_message, after extracting url_info, extract messageId from the URL and check it against self.processed_message_ids before processing.
• If the message ID has already been seen, log a debug message and return early — skipping AI generation and reply.
• To prevent unbounded memory growth, the set is cleared when it reaches max_processed_ids entries.

Testing

• Verified that the messageId parameter is present in reminderUrl for standard chat messages.
• The fix is minimal and does not affect messages lacking a messageId (they are still processed normally).
• No new dependencies introduced.