feat: add OAuth2 Client Credentials auth type

[steve-wecompare]

Summary

• Adds oauth2_client_credentials auth type for machine-to-machine OAuth2 flows
• Uses grant_type=client_credentials (no refresh token needed)
• Fields: Client ID, Client Secret, Token URL, Scope (optional)
• Use case: Amazon Creators API, Azure AD, and other M2M APIs

Changes (6 files, ~150 lines)

File	What
oauth2.ts	New getOAuth2ClientCredentialsToken() with in-memory token caching
gateway.ts	Handler to auto-inject Bearer tokens
auth-methods.ts	Added to VALID_TYPES + credential hint
auth-method-fields.svelte	UI form fields for add + edit
+page.server.ts	Server-side form parsing for add + edit
+page.svelte	Label mapping

Test plan

• Add OAuth2 Client Credentials auth to a target via the UI
• Verify token is fetched and cached on first proxied request
• Verify cached token is reused for subsequent requests
• Verify token is refreshed after expiry
• Edit the auth method and verify fields update correctly

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