fix: ensure digest notification sent even when bot not in private channel

[prabel]

Fix issue where digest notification was not being sent when incident was created from a private channel where the bot is not a member. Previously, the confirmation message posting would fail with "channel_not_found" error, which would prevent the digest notification from being sent.

Root Cause:

• Incident created in Notion successfully
• Confirmation message posting failed (bot not in private channel)
• Exception bubbled up to outer catch block
• Digest notification code never executed
• Leadership not notified of new incident

Solution:

• Wrap confirmation message posting in try-catch block
• Add fallback: send DM to user if channel posting fails
• Make Notion update non-blocking (wrapped in try-catch)
• Ensure digest notification always executes

Benefits:

• Digest notification ALWAYS sent when incident created
• User gets helpful DM explaining bot needs to be in channel
• Better error handling and logging
• Incident creation fully non-blocking
• No data loss or missing notifications

Changes:

• Add try-catch around confirmation message posting
• Add DM fallback with explanatory message
• Make updateNotionPageWithSlackInfo non-blocking
• Add detailed logging for each error scenario
• Fix TypeScript optional chaining for slackMessage.ts

Tested Scenarios:

• Bot in public channel: All messages sent ✅
• Bot in private channel: All messages sent ✅
• Bot NOT in private channel: Incident created, digest sent, user gets DM ✅

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