fix(create-post): guard DMs and ignore source notification failures

[Twest2]

What this changes

This tightens up the post-move flow in src/cogs/create_post.py:

• skips DM and group DM messages before the support-move logic runs
• makes the message sent back to the source channel optional, so a permissions issue there does not break the move after the support thread is already created

Why

The previous behavior had two edge cases needing to be addressed:

• a bot mention in a DM could reach the support-move branch and crash when the code tried to read guild roles
• if the bot could not send a follow-up message in the original channel, the move could fail partway through even though the new support thread had already been created

Validation

• python3 -m compileall -q src
• git diff --check

[Twest2]

@copilot review

[Copilot]

@copilot review

Reviewed the PR changes in fed609b and re-validated the branch. Current workflow run is still in progress with no failed jobs so far, and local validation (python3 -m compileall -q src, git diff --check) passes. No additional code changes were needed from this review pass.