Use a local SQLite3 backend instead of PostgreSQL Docker container.

* The provided SQLite3 database contains the required schemas, but no data and can be reset to if required.
* Remove all references to PostgreSQL in documentation and configuration.
* Replace native sqlite3 command with a console app to remove dependency on SQLite3 installation.

show-posts/Program.cs

@@ -0,0 +1,4 @@
+using table.lib;
+
+using var context = new PostsDbContext();
+Table<Post>.Add(context.Posts.ToList()).ToConsole();