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

In der Issue werden keine Dateien oder Tests genannt. Beginne damit, die kürzlich hinzugefügte languages table sowie das Schema oder die Modelle zu finden, in denen Projekt-Credentials und Provider-Strings gespeichert werden. Als erledigt gilt die Aufgabe, wenn unterstützte Provider eine globale Quelle haben, Credentials darauf verweisen und vorhandene Provider-Werte konsistent behandelt werden.