Provider: Initialization

Is your feature request related to a problem?

Provider names are currently stored as plain strings in the database. This means there's no single source of truth for valid
providers, leading to potential inconsistencies and making it harder to manage providers across organizations.

Describe the solution you'd like

Add a provider table to the global schema (similar to the recently added languages table). This would:

• Define all supported providers in one place
• Allow organizations to add credentials to their project referencing this table via foreign key
• Replace free-text provider strings with proper references, ensuring data consistency
• Make it easy to onboard new providers once and have them available across all orgs

この issue ではファイルやテストが指定されていません。まず、最近追加された languages table と、プロジェクトの認証情報および provider 文字列が保存されている schema または models を見つけます。対応している provider に単一のグローバルなソースがあり、認証情報がそれを参照し、既存の provider 値が一貫して扱われていれば完了です。