FEATURE: persistent key-value storage for AI Artifacts

[SamSaffron]

Introduces a persistent, user-scoped key-value storage system for
AI Artifacts, enabling them to be stateful and interactive. This
transforms artifacts from static content into mini-applications that can
save user input, preferences, and other data.

The core components of this feature are:

1. Model and API:

   • A new AiArtifactKeyValue model and corresponding database table to
     store data associated with a user and an artifact.
   • A new ArtifactKeyValuesController provides a RESTful API for
     CRUD operations (index, set, destroy) on the key-value data.
   • Permissions are enforced: users can only modify their own data but
     can view public data from other users.

2. Secure JavaScript Bridge:

   • A postMessage communication bridge is established between the
     sandboxed artifact iframe and the parent Discourse window.
   • A JavaScript API is exposed to the artifact as window.discourseArtifact
     with async methods: get(key), set(key, value, options),
     delete(key), and index(filter).
   • The parent window handles these requests, makes authenticated calls to the
     new controller, and returns the results to the iframe. This ensures
     security by keeping untrusted JS isolated.

3. AI Tool Integration:

   • The create_artifact tool is updated with a requires_storage
     boolean parameter.
   • If an artifact requires storage, its metadata is flagged, and the
     system prompt for the code-generating AI is augmented with detailed
     documentation for the new storage API.

4. Configuration:

   • Adds hidden site settings ai_artifact_kv_value_max_length and
     ai_artifact_max_keys_per_user_per_artifact for throttling.

This also includes a minor fix to use jsonb_set when updating
artifact metadata, ensuring other metadata fields are preserved.