Drop the redundant Post.Text/internalPost.Text fields and complete the
rename in server converters and test fixtures so the tree builds again.
- types.go: remove Text from Post and internalPost; toPost only writes
Author when the account acct is present
- posts_convert.go: apPostsPtrToItems / apPostsToAnalyticsItems read Content
- server tests: drop duplicate Text literal fields (values identical to Content)