OA-T4: Create OAuth Error Types (RFC 6749)

[devonartis]

Plan ref: Task 4, Milestone 2\n\nCreate internal/oauth/errors.go with error codes + WriteOAuthError. RFC 6749 §5.2 JSON format, not RFC 7807.\n\nFiles: internal/oauth/errors.go, internal/oauth/errors_test.go\nGate: go test ./internal/oauth/... -v -run TestWriteOAuthError