Model Config: CRUD endpoints and completion_type as array #891

Describe the current behavior
A clear description of how it currently works and what the limitations are.
model_config.completion_type is a single enum — one model can only support one type. To use gemini-2.5-flash for both text and stt, you'd need duplicate rows, but (provider, model_name) is unique. Also, model_config API is read-only — adding/editing models requires a migration + deploy.

Describe the enhancement you'd like
A clear and concise description of the improvement you want to see.

1. Convert completion_type to an array so a row can list multiple supported types.
2. Add POST (single + bulk), PATCH (single + bulk), DELETE on /api/v1/models.
3. Centralise Provider, CompletionType, Modality as StrEnums in app/models/llm/constants.py.
4. Validate input_modalities / output_modalities against the Modality enum.

Why is this enhancement needed?
Explain the benefits (e.g., performance, usability, maintainability, scalability).

• One row per model regardless of supported types — no workarounds.
• Add new models / tweak pricing without a deploy.
• Single source of truth for enums; invalid modality values rejected at the API boundary.