fix(firehose): align indexers and tests with richtext content schema (#162)
The lexicon source updated content from plain string to
{ $type, value } richtext object and renamed topic createdAt to
publishedAt, but the firehose indexers and tests still used the
old format. Docker builds from source, causing type errors that
blocked staging deploys.
- Extract content.value for sanitizeHtml calls in topic/reply indexers
- Use 'markdown' literal for contentFormat (derived from $type)
- Use record.publishedAt for topic timestamps
- Update all unit and integration test fixtures to match new schema
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