refactor: introduce CredentialRepository and ProviderRepository to ce…

[beubax]

Refactor to centralize storage and management logic; move functions into their respective domains

Description

Refactored the authsome server boundary so identity, vault, and auth remain more orthogonal, with server code coordinating them.

Key changes:

• Moved credential persistence into CredentialRepository.
• Moved provider resolution into ProviderRepository.
• Kept server-owned credential key construction inside credential_repository.py.
• Moved generic auth flow input/planning helpers into auth.utils.
• Moved server business records into server.schemas.
• Updated routes, dependencies, and tests to use the new server orchestration shape.

Motivation and Context

AuthService had accumulated too many responsibilities: provider loading, vault key construction, credential persistence, auth flow orchestration, and server policy behavior. This refactor narrows its role to orchestration while keeping storage and provider management in server-owned repositories.

This makes the module boundaries cleaner and prepares auth, identity, and vault to remain reusable independently.

How Has This Been Tested?

Tested with:

uv run pytest
uv run ruff check src/ tests/
uv run ty check src/
uv run pre-commit run --all-files

Result: all tests passed, including the full suite of 361 tests.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• All new and existing tests passed.