feat: add configurable RAG context retrieval settings for assistants

[gdmrino]

Changes

Add per-assistant settings to control how much knowledge is retrieved during RAG.
Users can choose between four modes:

• Default (50% of model context)
• Automatic relevance-based filtering (using autocut from v1)
• Custom percentage of context window
• Fixed number of chunks

Backend:

• New RagContextType enum and rag_context_type/rag_context_value fields
  on the Assistant domain entity, API models, DB table, and repository
• _get_retrieval_params() method replaces hardcoded chunk calculation
• Thread autocut_cutoff through ReferencesService for relevance mode
• Alembic migration adding columns to assistants table

Frontend:

• New RagContextSettings Svelte component with mode selector and
  value inputs (percentage slider, chunk count, or auto-relevance)
• Wired into the assistant edit page with change tracking and revert
• i18n strings for English and Swedish
• TypeScript schema types updated for the new fields

Why

Previously, retrieval during RAG was fixed to use 50% of the model context window, which did not provide enough flexibility for different use cases. This was especially limiting when working with open or smaller-context models, where consuming half the context for retrieval can significantly reduce space available for the actual response.

Testing

Tested both backend and frontend behavior:

Backend:

• Verified new database fields are created correctly via migration
• Confirmed each RagContextType produces the expected retrieval parameters
• Ensured default behavior remains unchanged for existing assistants
• Validated relevance mode correctly threads the autocut cutoff through retrieval

Frontend:

• Confirmed all four modes render and switch correctly in the assistant editor
• Verified percentage slider, chunk count input, and relevance mode settings persist after save
• Checked revert/change-tracking behavior works as expected

End-to-end:

• Created assistants using each mode and confirmed retrieval size matches configuration during queries

Screenshots