Add web tests covering the authorize endpoint, token exchange, PKCE
verification, single-use codes, and refresh token rotation. Add unit
tests for redirect URI validation and PKCE. Add E2E test for the full
browser-based authorization code flow with login redirect.
Extract setupApiUrl helper for E2E tests to avoid duplication.
Add POST /api/v1/oauth/token supporting authorization_code and
refresh_token grant types. Validates PKCE, exchanges codes for
JWT access tokens with refresh token rotation. Uses the shared
RefreshSession helper for the refresh grant.
Add POST /api/v1/oauth/authorize behind auth middleware. Validates
OAuth parameters (response_type, redirect_uri, PKCE), fetches the
authenticated user, creates an authorization code, and returns it
as JSON for the frontend to handle the redirect.
Add redirect URI validation that allowlists vikunja-* custom protocol
schemes, rejecting http/https and dangerous schemes like javascript:.
Add PKCE S256 verification following RFC 7636.
