Implement a model for storing the various bookmarks that are to be "remembered"

[krschacht]

From this Discussion #450

• Some model such as Post (or whatever we settle on)
• langchainrb_rails added and linked to this new model
• pg_search added and linked to this model
• All the search methods for this model in a concern (e.g. include Searchable for posts/searchable.rb)

I think success for this PR will be fixtures that create some Post and successfully search for them. Basically:

concerns/searchable_test.rb
Unit tests that call the various search methods (e.g. via keyword and via vector search)

One notable thing is that I believe generating the vectors will require calling the OpenAI API, and they probably get stored in some extra database table that langchainrb_rails creates. We should probably pre-create these vectors as fixtures since we won't want the tests to call the APIs.