Posts can carry images now, and the server stores and serves them itself

A new endpoint, /uploadmedia, accepts an image from a signed-in user -- up to 2MB, sent base64-encoded in the request body with the content type as a parameter -- stores it in a new media table in the database, and answers with the address it will be served from: /media/1, /media/2, and so on, permanent ids just like posts. Request that address and the image comes back byte-for-byte with the right content type. No filenames, no image processing, no separate file storage -- the picture lives in the same database as everything else, which means the same one-file simplicity on SQLite servers, and the same backup story: the export and import verbs carry the media table along (the bytes travel as text inside the JSON), so a migrated server keeps its images with their addresses intact. The table is called media rather than images on purpose -- one binary table can someday hold audio and video too, and the type column already tells them apart. MySQL servers get the table definition in installMysql.md. The upload limit is a config setting, maxMediaUploadBytes, 2MB by default. Under the hood, davesql (v0.7.1) learned to write binary values on both engines -- the last piece the feature needed. (Server v0.6.1.)