Reposting failed RSS feed entries

When posting an RSS entry fails for any reason, the entry should be treated as "not seen", the next time that the feed is checked.

**Context:**
Sometimes an entry is missing and there is not an error message in the bot config page. I see this across 5 :matrix.org rooms. Rooms with the same feed have the same missing entries, i.e. either both have a message or they both do not. So this could possibly be bot-side rather than Matrix-side.

Now my point is resiliency. I do not care why failure happens. But when it happens, please behave as if the entry were not seen. The feed is checked again after X minutes and the missing entry is still in the RSS. So it has a chance of being posted on the next cycle.

Start by tracing the RSS feed check and posting flow to see where an entry is marked as seen and how posting failures are handled. Done means a failed RSS entry remains eligible for the next feed check, allowing it to be posted while preserving the existing retry cycle.

Written by the indexing model from the issue text.